0:00:00hi
0:00:01let's talk about electromagnetic radiation
0:00:04electromagnetic radiation
0:00:06is the wave like property
0:00:08it has alternating magnetic and electric fields
0:00:11now we're not gonna talk about the electric and magnetic fields too much but the
0:00:14wave nature is important to us what are the properties of ways
0:00:18well while family with waves event at the beach we've seen waves come in from
0:00:23the ocean
0:00:25one of the properties
0:00:26well there's a speed of the wave moves
0:00:29if you see a search for any catches the wave
0:00:32the speed that the server moves writing the class pressed
0:00:36is the speed of the way
0:00:39we write that down
0:00:40in on the board as a sine wave
0:00:44that will designate our wave like property and if it travels
0:00:48in the
0:00:49certain direction will give that direction of travel
0:00:52and we'll give it is seen
0:00:54when we talk about electromagnetic radiation
0:00:56it moves at the speed of light
0:00:58as opposed to the speed of server
0:01:01so this mean is the speed of light
0:01:05what other properties the waves have
0:01:07well there's quality the separation between the crest of the way
0:01:12if you see one server catch a wave and another server catch the wave right
0:01:16behind him
0:01:17the separation between those two surfers
0:01:20as they approach the beach is the wave like
0:01:22and their speed will be equal and their distance will stay about the same
0:01:28their distance between and the wavelength
0:01:31the length between the crest
0:01:33is given the symbol lander and is the wavelength of the way
0:01:37an example seven hundred an animator wave like
0:01:40that wavelength corresponds to a wavelength in the visible region of the electromagnetic spectrum visible
0:01:46light
0:01:47and often use the term light and electromagnetic radiation interchangeably
0:01:52seven hundred nanometres when that wreck electromagnetic radiation strikes are i appears reddish
0:01:59electromagnetic radiation with wavelength four hundred nanometres appears polish
0:02:03so the different wavelengths have different properties
0:02:06and for some electromagnetic radiation are sensitive to them
0:02:10so
0:02:10we have different colours for different wavelengths
0:02:14another property of ways are the frequency
0:02:17the frequency the way it is how often a wave crest past you
0:02:21so you have
0:02:22three wave crests past you per second
0:02:26the frequency would be one over three or one third of the second
0:02:31now we say reciprocal seconds one over three seconds we give that
0:02:35a special unit called hz
0:02:37one third of our hz
0:02:39is
0:02:39one over three seconds
0:02:42the way blank the frequency
0:02:45always multiplied together to giving the speed
0:02:48product of the wavelength in the frequency is the speed
0:02:51and for electromagnetic radiation that's the speed of light
0:02:54three times ten to the eight meters per second
0:02:57we're gonna talk about weight is electromagnetic radiation and light
0:03:00all the time in discourse so we should be very familiar with the relationships between
0:03:04electromagnetic radiation frequency wavelength and speed
0:03:12electromagnetic radiation encompasses a broader range of wave like some frequencies
0:03:17from
0:03:18astronomical proportions ten to the eight meters like
0:03:21for the sun distances
0:03:23down to
0:03:24tiny microscopic
0:03:25with like stand of the minus sixty meters
0:03:29the range is have different names basically historical
0:03:33names
0:03:34describing the kinds of radiation in each band
0:03:37so
0:03:38the long wavelengths radio waves
0:03:42microwave slightly shorter in wavelength still though
0:03:45in proportions we could
0:03:46resolve
0:03:47few meters to a few centimetres
0:03:50infrared radiation now down below are
0:03:53range of resolving it
0:03:55and visible ultraviolet radiation
0:03:58x rays and gamma rays
0:04:00all these flavours of radiation are emitted by the sign so during solar of ends
0:04:06the proof is bayes by all these kinds of radiation and there's other sources of
0:04:10these radiation obviously we can make microwaves in radio waves
0:04:14infrared waves and visible ways we have flashlight with microwave ovens we have meters we
0:04:19have radios
0:04:21so all those things are were able to
0:04:24produce electromagnetic radiation
0:04:27the visible spectrum in particular in between the infrared and ultraviolet we're gonna look at
0:04:32a lot
0:04:32these are the wavelength that our allies are sensitive to
0:04:35from four hundred to seven hundred ninety meters approximately
0:04:39so blue green yellow or orange and red light
0:04:42we can detect
0:04:44and these wavelengths stimulate a rise and those signals travel to our brain we perceive
0:04:50different colours
0:04:51the combination of all those colours
0:04:53altogether would give us a white light
0:04:56so
0:04:57from blue to read
0:04:59completely all at once
0:05:00you would perceive white light
0:05:03but you can take one light and break it back down into its component colours
0:05:07you can do that with
0:05:09putting obstacles in the way putting a grading in the way of the light
0:05:12so the light
0:05:14of different way likes interact differently with that grading or those obstacles so long waves
0:05:19wave like sort of reacted differently than short wavelengths
0:05:22you can spread out
0:05:24the wavelength in space
0:05:26we can actually do that diffraction grating tsar cheap and easy to make
0:05:29there's actually some in these classes here
0:05:31what i'm gonna do is undertake this white light source
0:05:36i'm gonna turn that on and we'll put on these the fraction glasses and will
0:05:39be able to see when we look at the light
0:05:45it resolved into
0:05:48blue green
0:05:50orange red it's component colours
0:05:53so this is a beautiful example of the electromagnetic spectrum
0:05:56white light being resolved into its component wavelength
0:05:59by a diffraction grating
0:06:02we talking a lot about
0:06:04wavelengths
0:06:05and visible light during the scores
0:06:07so this experiment with the diffraction grating showing separation of wavelengths
0:06:12is very useful for understanding of light
0:06:17when you talk about electromagnetic waves the waves are properties of an electric and magnetic
0:06:21field
0:06:23and they're
0:06:23they can be very long like radio waves what i can be very short like
0:06:27gamma waves
0:06:28but
0:06:29there's no sense that we have they can perceive then we can see them are
0:06:32smell them are touch them or understand the wave behavior
0:06:36when we look at ocean waves it's obvious that there is a wave property and
0:06:39we can see their speed and they're
0:06:42wavelength and their frequencies
0:06:44well electromagnetic radiation you can't
0:06:47so be nice to be able to demonstrate
0:06:49the wave property in experiment
0:06:52well with ocean waves waterways it's well known windows waves hit obstacles how they behave
0:06:59so
0:07:00it be need to arrange an experiment where electromagnetic wave hit some obstacles and display
0:07:05those same kind of wave characteristics
0:07:07so that's what we're gonna do
0:07:09so consider this we're gonna take us let a tiny little hole poked in a
0:07:13screen and shine light through it and then look far away
0:07:18at a wall or some other screen without like it's and c with that looks
0:07:22like
0:07:22so i think you understand as you take this is like
0:07:25poking a hole in a pie plate and shining a flashlight through it on will
0:07:29wall
0:07:30you know what you'd see you see
0:07:31an image of the whole that you poked and it be diffuse around the edges
0:07:35and it be bright in the centre
0:07:37that's because the light is distracting in spreading out
0:07:40around the edges of the a whole you pop
0:07:44that's exactly what are drawn here i have a light passing through a
0:07:49a whole
0:07:50at a screen far away
0:07:52and i've applied in the intensity distribution
0:07:55now this is monochromatic lights only one wavelength involved it's not like a flashlight
0:07:59so the more like a laser
0:08:01going through a slit
0:08:03so
0:08:04what you have the always the same thing a bright spot beginning do at the
0:08:07edges live pointed out here
0:08:10so this intensity distribution is just like
0:08:13a probability distribution if it were particles
0:08:16that is
0:08:17let's say i shot bullets or babies or something through a whole
0:08:21some of them would take off the sides here in a spread out in a
0:08:25and out of the fringes but a lot i will go right through
0:08:28and hit directly across from the whole
0:08:30so you'd have a high probability of by a particle hitting directly across from the
0:08:35whole
0:08:35and slightly lower probability as you move away from the whole
0:08:40so let's look at another slit we could do the same thing with light
0:08:43a second slip and have the same kind of experience
0:08:46where a lot of particles are a lot of waves are a lot of beams
0:08:50it directly across from the slit
0:08:53so to sleep together is where gets interesting
0:08:56when waves interact
0:08:58that's how we can tell their way
0:09:00and you probably know this by looking at waves in the bathtub or waves on
0:09:03the beach
0:09:04if there's something in the middle and waves hit them
0:09:08there's a pattern
0:09:09that's
0:09:11predictable as those waves moves away from the article
0:09:13so this is like an obstacle to
0:09:15to light waves
0:09:16and we'll talk about like in the visible spectrum so you can actually see it
0:09:21so two slits
0:09:23but like passes through them
0:09:25what you might expect first as well you get a bright spot across from one
0:09:29slit and a bright spot across from the other split
0:09:32and it turns out for some wavelength in the first
0:09:36orientation of this leads that's what you get
0:09:38but if you range the slit appropriately
0:09:41you get a very distinct patterns
0:09:43instead of
0:09:44two
0:09:45strong intensities
0:09:47across from slit a and slid be
0:09:50what you actually see
0:09:51is
0:09:52eight
0:09:53had and of intensities light and dark spots
0:09:57the art
0:09:58directly across from the slits
0:10:00so you see bright spot
0:10:02dark spot bright spot dark spot right but dark spot
0:10:05alternating away from the center
0:10:08of the slits the brightest spot is actually right between the slits
0:10:12so
0:10:13that doesn't look like a particle like property at all
0:10:16particles would never do this if you change your b b's
0:10:19or your
0:10:20ping pong balls or something macroscopic to these two holes you would get a bright
0:10:24spot here in a high probability spot here
0:10:27so this is a property only of weights
0:10:30and how can we understand it
0:10:32well it's actually pretty clear
0:10:35if you think about it always has to travel
0:10:38from the slits
0:10:39to the screen
0:10:42and if the light has to travel from this leads to the screen so here's
0:10:45a way of coming from each slit
0:10:48in the centre
0:10:49notice that the distance that this wave has to travel and the distance that wave
0:10:53has to travel will be the same
0:10:56so two waves travelling in units in the same distance
0:11:01will be in phase
0:11:03they will be either add up peak
0:11:05or a trough
0:11:07identically
0:11:08because they've travelled the same distance
0:11:11now what about it didn't spot up here
0:11:13well if you think about a wave travelling from this little in a way of
0:11:16travelling from this that ms wave now has a longer distance to travel
0:11:21then this way
0:11:24so the one that has the farther path is gonna be out of phase somewhat
0:11:29with the one that has the shorter pat
0:11:31so think about that one wave travels up down up down up the other one
0:11:36has a longer path
0:11:38up down up down up down
0:11:41so what a rise in an phase the other arrives at the down phase
0:11:46and when you add those two intensities together
0:11:49what you get is destructive interference
0:11:53so the waves added to give a zero intensity
0:11:57it's actually really cool and it's something you can see in the experiments that will
0:12:01show you later on we'll showing a laser through a slit
0:12:03and will actually show you this pattern of light and dark spots
0:12:07this shows us that electromagnetic radiation is actually a way
0:12:17when the demo level slightly wary set up a laser to slip interference pattern
0:12:22is set up a laser
0:12:23a set of two closely spaced slides
0:12:26in the screen where we can observe the alternating light and dark spots corresponding to
0:12:31constructive and destructive interference
0:12:36so let's think about this interference pattern of light in terms of a can quiz
0:12:41so if you science a green light
0:12:44and you
0:12:45ten this
0:12:45to slit interference pattern
0:12:48how that interference pattern change if you increase the wavelength lander
0:12:53so you go from say relayed to read like to longer wavelength light
0:12:58how the interference pattern change will be better get more compact look about the same
0:13:04or really get more dispersed
0:13:06think about that per second and make a selection
0:13:22let's consider three possible
0:13:24explanation for the three answers may increase wavelet causes higher frequency spacing since wavelength and
0:13:31frequency are inversely proportional
0:13:34or be the interference depends only on the spacing between the slits
0:13:37so increasing the wave like doesn't change anything and no change that occur
0:13:41or c
0:13:42longer wavelength white causes destructor interference a larger intervals
0:13:47along the wall
0:13:48think about those three options and make another selection
0:14:04when you change the wavelength
0:14:06in a to slit experiment what do you expect will happen
0:14:09well
0:14:10what we know from artist let wave interference pattern is
0:14:14two slits "'cause" in a high intensity then low intensity because the
0:14:19path are equal
0:14:21right between the two slits
0:14:23but if you go to the first peak say then one way as to travel
0:14:28farther than the other
0:14:29and it's that
0:14:31extra distance
0:14:32because is either constructive a bright spot or destructive a didn't spot
0:14:38interference
0:14:39on the screen
0:14:41so but a larger
0:14:43the wavelength
0:14:44the larger the spacing will be
0:14:46that is you have to move farther up the screen
0:14:50to make this path like long enough to get it extra half wavelength
0:14:56and give you
0:14:57constructive or destructive interference up a screen
0:15:00so longer wavelength means the spreading out
0:15:03of the diffraction pattern
0:15:05on the screen and the correct answer is c
0:15:10so let's think about this interference pattern of light in terms of a can quiz
0:15:15so if you shanks a green light
0:15:18and you
0:15:18ten this to slit interference pattern
0:15:22how that interference pattern change if you increase the wavelength lander
0:15:27so you go from say relayed to read like longer wavelength light
0:15:32how the interference pattern change will be better get more compact will look about the
0:15:37same or really get more dispersed
0:15:40think about that per second and make a selection
0:15:55let's consider three possible
0:15:58explanation for the three answers may increase waveline causes higher frequency spacing since wavelength and
0:16:05frequency are inversely proportional
0:16:07or be the interference depends only on the spacing between the slits
0:16:11so increasing the wave like doesn't change anything and no change whittaker
0:16:15or c
0:16:16longer wavelength white causes destructor interference a larger intervals
0:16:21along the wall
0:16:22think about those three options and make another selection
0:16:38when you change the wavelength
0:16:40and a to slit experiment what do you expect will happen
0:16:43well
0:16:44what we know from artist let wave interference pattern is
0:16:47two slits "'cause" in a high intensity then low intensity because the
0:16:53has are equal
0:16:55right between the two slits
0:16:57but if you go to the first peak say then one way as to travel
0:17:02farther than the other
0:17:03and it's that
0:17:05extra distance
0:17:06that causes either constructive a bright spot or destructive a didn't spot
0:17:12interference
0:17:13on the screen
0:17:15so we larger
0:17:17the wavelength
0:17:18the larger the spacing will be
0:17:20that is you have to move farther up the screen
0:17:24to make this path blank long enough to get it extra half wavelength
0:17:30and give you
0:17:31constructive or destructive interference up a screen
0:17:34so longer wavelength means the spreading out
0:17:37of the diffraction pattern on the screen and the correct answer is c
0:17:44when electromagnetic radiation interacts with matter that radiation can be changed or it can be
0:17:50emitted
0:17:51or it can be absorbed in some way
0:17:53this is a really important phenomenon chemistry because we can see atoms and molecules with
0:17:58a rise
0:17:59in fact
0:18:00when you see on t v a scientist
0:18:01they'll always have
0:18:02a lab coat to have some safety glasses
0:18:05and
0:18:05regardless what kind assigned just as being portrayed the always have a microscope
0:18:09and when one to tell you something about an atom or molecule they look to
0:18:13their microscope in a cell without will default you'll lose
0:18:15that is just crazy
0:18:17we can see there's no optical microscope they can result atoms and molecules
0:18:22what we do is we have radiation of various wavelengths interact with atoms and molecules
0:18:28and we'd dues things about the molecules based on how those atoms and molecules interact
0:18:33with the radiation
0:18:35so we're radiation it's a molecule or atom or any kind of matter
0:18:39many things can happen it can be absorbed it
0:18:41radiation can be admitted by excited atoms relation can change
0:18:46from high frequency to low frequency radiation there can be a reflection process
0:18:50all kinds of different things that help us understand the matter
0:18:54so let's talk about absorption and emission
0:18:57i can happen in many different ways
0:18:58you can have a continuous absorption
0:19:01so you can have
0:19:02continuous absorption
0:19:03of many wavelengths
0:19:06so a band of wavelengths different colours hitting you all at once
0:19:10so for instance when we see white light that's all the colours mixed together coming
0:19:14at us
0:19:15or we can see a book read
0:19:17or yellowish orange several different wavelengths
0:19:20coming out as it once
0:19:21for several different wavelength could be
0:19:24absorb at once
0:19:26so absorption and emission
0:19:28happen
0:19:29in continuous broad swaths of radiation
0:19:33and this is actually why you perceive colour
0:19:36when white light a combination of all the colours it's an object
0:19:40some of those colours can be absorbed
0:19:43the colours that aren't absorb passed through
0:19:46or reflected back and pitch your bibles
0:19:49and the wavelength that hit your eyeballs can be either red or blue or re
0:19:53for instance the screen looks blue because
0:19:57wavelengths of white light are hitting it
0:19:59but only the blues are coming back and hitting your mobile
0:20:04now
0:20:05absorption and emission can also be discrete or line base that is specific wavelengths are
0:20:10absorbed
0:20:12so you can broadcast
0:20:14or
0:20:15have a lot of radiation hitting an object in a broadband
0:20:18but only sort in wavelengths are absorbed and removed from the spectrum
0:20:23that's how that would look now you could also have that intonation you could have
0:20:26only certain
0:20:28wavelengths emitted from an excited atom or molecule are excited matter
0:20:33again this would help you perceive colour of
0:20:36a blue and r
0:20:37green and the yellow
0:20:39wavelength were emitted from an object
0:20:41that object
0:20:42would appear
0:20:43greenish blue
0:20:45so we can actually look at absorption and emission from atoms and i can show
0:20:49you a continuous
0:20:52emission spectrum it's actually
0:20:54several lines being emitted at once
0:20:57from atoms
0:20:59so let's look at that
0:21:00a but on my safety glasses so we can do an experiment safely if you're
0:21:03on the desktop
0:21:04and i'm gonna bring in
0:21:09a series of tulips filled with
0:21:12now there's various different gases here and i can excite these gases with an electric
0:21:16current
0:21:17and then they'll unit
0:21:19electromagnetic radiation back to you in the visible region so you can see
0:21:24a luxury son there aren't in the visible region
0:21:26but we can see them so it doesn't matter
0:21:29what i'm gonna do i'll take an electric
0:21:31current here a little stimulator
0:21:33and
0:21:35stimulate these atoms of gas and you should see my visible radiation coming out of
0:21:40the two
0:21:45so there it is
0:21:51she is a decision from side as
0:21:58in the visible range
0:22:00that's a really interesting experiment it shows
0:22:03when a line spectrum is invaded
0:22:06and there are several lines that ones
0:22:08you'll see kind of a bluish green or a purplish colour
0:22:11let's look at absorption
0:22:17here i have some coloured solutions
0:22:18and wireless solutions colour well white light is hitting them
0:22:23and when the white light hits them
0:22:24several wavelength are absorbed
0:22:26all their wavelength passed directly through and he chewing the eyeball
0:22:30so red wavelength
0:22:32passed through a hit you in the eyeball for the solution and wu wavelengths
0:22:36other colours absorb but who passes through
0:22:40and hit you when the eyeball
0:22:42so we're say
0:22:43all colours but read are absorbed by this
0:22:47can i prove that in a different way
0:22:49well
0:22:51here's a red pen laser
0:22:54if i shine this red pen laser under my hand
0:22:57i think it can see it there
0:23:00now what i'm gonna do is i'm gonna shine this red pen laser through the
0:23:04red solution
0:23:05and i'm telling you and i think you probably agree with me that the red
0:23:09should pass right through because this solution
0:23:11does not absorb read
0:23:12reds hitting your eyeballs now
0:23:14so this red pen laser now passing through the solution
0:23:19still hits my head
0:23:23but i told you this blue solution it must be absorbing reds or you be
0:23:27seen some rad
0:23:29so i should be able to take my red pen laser
0:23:33here it is just on my hand again
0:23:35and passed through this blue solution
0:23:38and have the red pen laser absorb so let's do that
0:23:41here it is hitting just my hand and i'm gonna move back to like go
0:23:44through the solution and you can see
0:23:47the red is now absorbed by that blue solution
0:23:52that's a beautiful demonstration of
0:23:54a discrete
0:23:55a single wavelength align absorption
0:23:59by a blue solution
0:24:01that's how colours work
0:24:02absorption and emission we're gonna talk about a lot in discourse because they help us
0:24:06understand matter
0:24:10so let's think about this interference pattern of light in terms of a can quiz
0:24:15so if you science a green light
0:24:18and you
0:24:18ten this
0:24:19to slit interference pattern
0:24:22how that interference pattern change if you increase the wavelength lambda
0:24:27so you go from say relayed to read like longer wavelength light
0:24:32how the interference pattern change
0:24:34will be better get more compact will look about the same or really get more
0:24:39dispersed
0:24:40think about that per second and make a selection
0:24:55let's consider three possible
0:24:58explanation for the three answers may increase wavelet causes higher frequency spacing since wavelength and
0:25:04frequency are inversely proportional
0:25:07or be the interference depends only on the spacing between the slits
0:25:11so increasing the wave like doesn't change anything and no change that occur
0:25:15or c
0:25:16longer wavelength white causes destructor interference a larger intervals
0:25:21along the wall
0:25:22think about those three options and make another selection
0:25:38when you change the wavelength
0:25:40in a to slit experiment what do you expect will happen
0:25:43well
0:25:44what we know from artist let wave interference pattern is
0:25:47two slits "'cause" in a high intensity then low intensity because the
0:25:53path are equal
0:25:55right between the two slits
0:25:57but if you go to the first peak say then one way as to travel
0:26:02farther than the other
0:26:03and it's that extra distance
0:26:06because is either constructive a bright spot or destructive a didn't spot
0:26:12interference
0:26:13on the screen
0:26:15so the larger
0:26:17the wavelength
0:26:18the larger the spacing will be
0:26:20that is you have to move farther up the screen
0:26:24to make this path like long enough to get it extra half wavelength
0:26:30and give you
0:26:31constructive or destructive interference up a screen
0:26:34so longer wavelength means the spreading out
0:26:37of the diffraction pattern on the screen and the correct answer is c
0:26:46when the demo level slightly wary set up an experiment
0:26:50to show absorb ins
0:26:52coloured solutions is broken up the white light into its component colours and when you
0:26:57place is a red solution in the past of the white light
0:27:00notice that read it is transmitted
0:27:03but green and blue are absorbed
0:27:06now a yellow solution
0:27:08when the yellow solution is placed in the path of the white light
0:27:12we'll see that the blue is strongly absorbed
0:27:15and the combination of red and green
0:27:18is what causes the yellow colour
0:27:21finally
0:27:22we have a blue solution
0:27:25the blue solution
0:27:27should absorb
0:27:28the red and the green and transmit the blue
0:27:32so here we see the origin of colours
0:27:35colours are absorbed and transmitted resulting in the colours that we observe thank you honey
0:27:43so let's talk about absorption of light in terms of it can quiz
0:27:47if you have an object that has an absorption spectrum that looks like this what
0:27:51color is that object
0:27:53is it blue green or red
0:27:55think about that for a minute and make a selection
0:28:10but continue argument for each of the possible answers
0:28:14answer a blue light passes through the filters of the article your blue
0:28:18answer be a filter that absorbs read and passes blue or making an object appeared
0:28:22green
0:28:23or
0:28:24red line is absorbed by the filter so the object will appear red
0:28:28think about those three possible solutions for a minute and make another selection
0:28:44and object that has
0:28:45and absorption spectrum that looks like this
0:28:47is absorbing strongly in the reds and yellows
0:28:50and passing
0:28:53wavelength and the blue so we wavelengths will pass through
0:28:57strike your i and this object
0:28:59will appear blue
0:29:04let's look at a couple filters and how they'll affect a blue object
0:29:08so three possible filters
0:29:11put in front of a blue object
0:29:12which one of them
0:29:13would make the object appear black
0:29:15think about that for a minute
0:29:17and make a selection
0:29:31let's look at some possible explanations for each option
0:29:36filter one the passes blue light and by the way
0:29:38which combine to form black
0:29:41or option be filter to absorb the blue light from the blue object making it
0:29:45appear black or
0:29:47filter three removes violet and ultraviolet light making the object appear black
0:29:51think about those and make another selection
0:30:08so we've got a little object
0:30:09we're gonna put filters between the blue object and us and we're gonna see what
0:30:14we see so
0:30:16a blue object
0:30:18wavelength the blue light
0:30:19are being emitted from that object that's white appears blue
0:30:22how do we make it appear black
0:30:24we have to remove that remaining blue wavelengths
0:30:27assumption removing all the wavelengths
0:30:29from that object so non here are i and we have a black object
0:30:35so we need to fine
0:30:37a filter that absorbs strongly in the blue
0:30:40and that is
0:30:41filter number two
0:30:42correct answer here
0:30:44we
0:30:47we all know glasses transparent
0:30:50so let's look at absorption over a broader range of electromagnetic spectrum
0:30:55transparent glass
0:30:56should have an absorption spectrum that looks like either one or two or three
0:31:00think about that for a minute and make a selection
0:31:16let's look an argument for each of the answers
0:31:19a transparent glass passes all visible wavelength
0:31:22and only absorbed above the u b
0:31:25or b
0:31:26transparent glass must absorb some like in the visible region
0:31:31or c transparent glass most absorb the entire visible region
0:31:35think about those alternatives
0:31:36and make another selection
0:31:50if the option spectrum are transparent glass is what we're talking about
0:31:54and we're talking about a rather broad region of electromagnetic spectrum from ultraviolet wavelengths down
0:32:00to infrared wavelengths and i've
0:32:03overlay the common visible coloured wavelengths
0:32:06in the centre
0:32:07so let's look at those three absorption spectra number one
0:32:11absorb strongly in the ultraviolet
0:32:13number to absorb strongly not the ultraviolet and
0:32:17some visible wavelengths
0:32:19an option three absorbs
0:32:22ultraviolet visible wavelengths
0:32:24looks like most of them and terminate somewhere in the i r so it also
0:32:30absorbs a little bit of infrared
0:32:32wavelengths as well but in order for the glass to be transparent
0:32:36all visible wavelengths must pass through
0:32:38transparent glass will pass white like directly through it you can see all colours to
0:32:43transparent glass
0:32:44so all colours must pass through
0:32:46so we have to choose one that has
0:32:48no absorption
0:32:50spectra in the visible range that is option one
0:32:53so the correct answer here
0:32:55is k
0:32:58but still calculation with electromagnetic radiation the properties of wavelet in frequency
0:33:04what frequency and designation of radiation with wavelength eight point eight three p commuters
0:33:09is emitted from technician
0:33:11ninety nine during its nuclear decay
0:33:14so
0:33:15we understand a wavelength
0:33:17a point eight three you can meters we can change that into a frequency
0:33:21knowing that the way
0:33:23waves travel at the speed of light c
0:33:26so the product of the wavelength in the frequency
0:33:29is the speed we can solve for the frequency
0:33:32frequency is the speed over the wavelength
0:33:35and just simply plug those two numbers in
0:33:38a point eight three and now i've written ten to the minus twelve meters
0:33:42because my
0:33:44speed of light is
0:33:45it has the units of meters per second so one all my units to be
0:33:48the same
0:33:49i can do that division three point four times ten to the nineteenth reciprocal seconds
0:33:54or
0:33:55three point o four three point four times ten to the nineteen her
0:33:59so very high frequency very short wave like those we'd expect what
0:34:05region of electromagnetic spectrum of that correspond to
0:34:07well
0:34:09this
0:34:10wavelength in p commanders firmly in the gamma wave region of the spectrum so we
0:34:15have
0:34:16gavel waves emitted from technician ninety nine during its nuclear decay
0:34:23but still calculation with electromagnetic radiation the properties of wavelength and frequency
0:34:29what frequency and designation of radiation with wavelength eight point eight three p commuters
0:34:34is emitted from technician
0:34:36ninety nine during its nuclear decay
0:34:39so
0:34:39we understand a wavelength
0:34:41eight point eight three p commuters we can change that into a frequency
0:34:46knowing that the way
0:34:48waves travel at the speed of light c
0:34:51so the product of the wavelength in the frequency
0:34:54is the speed we can solve for the frequency
0:34:57frequency is the speed over the wavelength
0:35:00and just simply plug those two numbers in
0:35:03a point eight three and now i've written tend of the minus twelve meters
0:35:07because my
0:35:09speed of light is
0:35:10it has the units of meters per second so one all my units to be
0:35:13the same
0:35:14i can do that division three point four times tend to the nineteenth reciprocal seconds
0:35:19or
0:35:20three point o four three point four times ten to the nineteen her
0:35:24so very high frequency very short wave like those we'd expect what
0:35:30region of electromagnetic spectrum of that correspond to
0:35:32well
0:35:33this
0:35:35wavelength in p commanders firmly in the gamma wave region of the spectrum so we
0:35:40have
0:35:41gavel waves emitted from technician ninety nine during its nuclear decay
0:35:48electromagnetic radiation is composed of wavelengths from very long the very short
0:35:54we've talked about the relationship
0:35:55between the wavelength the frequency and the speed of electromagnetic radiation
0:36:01in fact the product go
0:36:03wavelength and the frequency is the speed
0:36:05for electromagnetic radiation light
0:36:08speed is fixed at the speed of like
0:36:11so you have the wavelength increases the frequency has to decrease their inversely proportional
0:36:16and you can see i have
0:36:18wave like increasing here and frequency increasing here
0:36:23the visible region in particular we really talk a lot about because we can
0:36:27perceive the length of the radiation by the colour
0:36:31so we can make that easy connection between all wave
0:36:34and it's blank by the colour that we see
0:36:37long wavelength in the visible region
0:36:40are read
0:36:41intermediate wavelength going down from yellow to orange the green the blue
0:36:48indigo and violet
0:36:49from
0:36:50long to shortwave links
0:36:52in fact this kind of spells the guy's name role in g b are from
0:36:56long to shortwave likes i often write that down and i can remember the colours
0:37:00of the rainbow
0:37:01now there's more properties to electromagnetic radiation in waves in general
0:37:05for instance the intensity we haven't touched on that yet
0:37:08you can think of the intensity of waves in the ocean as their height as
0:37:12they come in
0:37:13to the shore
0:37:14a big way would be an intense way that all wave and intense way
0:37:20i do we do that for electromagnetic radiation
0:37:22well let's talk about intensity more as we go through this talk
0:37:27now the intensity of light or electromagnetic radiation maybe more of an intuitive property that's
0:37:33looking at this way
0:37:34if you have and intensity originally of
0:37:37i zero
0:37:38you passing through a filter and you can and intensity of half that i zero
0:37:42over two
0:37:44well with the intensity mean if you put an additional column in the past
0:37:47so i zero going through a
0:37:51pair of identical filters
0:37:54with that reduce the intensity down to acquire and or zero of the original intensity
0:38:00think about that for a minute and make a selection
0:38:15let's consider an argument for each of the possible answers
0:38:19hey
0:38:20a second filter reduces the intensity by one half again so you have a quarter
0:38:24of the original intensity
0:38:25be the second filter has twice the effect reducing the ten c by a factor
0:38:29of four so one at the original or option c
0:38:34the first filter removes half intensity so the second remove the remaining intensity giving you
0:38:38zero intensity
0:38:40think about those three possibilities and make a selection
0:38:55so we've been talking about intensity the brightness of light
0:38:59in terms of extenuating or reducing it with a filter
0:39:03so
0:39:03and original intensity reduced to half the brightness
0:39:08by a single filter
0:39:09what would be near fact
0:39:11and identical filter put in the path again
0:39:13well
0:39:14if the first filter reduces it by
0:39:16one half so it's half is bright here
0:39:19a second filter would make this
0:39:22half is bright
0:39:23so you go from have the original intensity to a quarter of the original intensity
0:39:27after two filters
0:39:29so the correct answer here is a one quarter the original intensity
0:39:35the intensity of light words brightness
0:39:38can be considered in a thought experiment
0:39:40just like we did with matter
0:39:41we said
0:39:42if i take a chunk of carbon
0:39:44solid matter and i cut in half and then in half again and again and
0:39:49again and continue cutting in and have to like get the tiniest piece of matter
0:39:53it still has the properties of carbon
0:39:56that timing is particle
0:39:58would be an atom
0:40:00can we do that with light intensity can we take a bright light
0:40:04and bring in filters that reduce the intensity by a factor of two
0:40:08cut the light intensity and have
0:40:10again and again and again and again and again
0:40:14well of course we can do the experiment
0:40:15what happens
0:40:17well if you're watching this experiment the light would get dimmer
0:40:22as each filter was added
0:40:25and they like what eventually didn't completely
0:40:28but then something interesting would happen
0:40:30you would see
0:40:31individual
0:40:33flashes of light
0:40:35it turns out here eyes are accustomed to the smallest unit of like
0:40:40individual pulses
0:40:42these individual pulses like particles of like just like chopping matter up into you get
0:40:47these smallest
0:40:48particle an atom
0:40:49you can reduce length intensity to you get the smallest particle of light
0:40:55in we call these particles of light these tiny flashes we call them photons
0:41:00and is four times carry the smallest amount of energy of light
0:41:05time
0:41:06is what we call it mean energy of the photon is given by h blocks
0:41:09constant
0:41:10times
0:41:11the frequency of the light
0:41:13so the frequency of a light times punks constant
0:41:17gives you the energy of the photon one of the most important
0:41:20equations in this class
0:41:22in fact if you take nothing else open can one
0:41:24take on the fact that the energy photons is given by
0:41:27blanks constant times their frequency
0:41:30once constant a very small number
0:41:32six point six two times ten to the minus thirty fourth
0:41:36jules seconds
0:41:38that tells you this incredibly tiny amount of energy
0:41:41but our eyes can detect that's the interesting thing
0:41:44you do this experiment in a dark room where your eyes are acclimated to the
0:41:48dark your pupils wide open
0:41:50you can actually reduce intensity you see the individual flashes afford that
0:41:55fascinating experiment to do
0:41:57now visual perception has been quantitative for a long time
0:42:01we've said that the limits of human perception are a candle
0:42:08on a clear dark night at thirty miles away
0:42:11that's when the intensity is at the level of individual photons hitting your i
0:42:17so
0:42:18light has a particle nature just like matter has a particle nature
0:42:23now we can detect that particle nature
0:42:26of electromagnetic radiation of all forms this we did a thought experiment on
0:42:32visible light
0:42:34we can take a gamma way
0:42:36remember that high frequency and now we know high energy
0:42:41light
0:42:43well
0:42:44here is a radioactive source
0:42:47this is radioactive c zero one thirty seven in admits
0:42:51gamma waves
0:42:52when it
0:42:53the case
0:42:54some particularly
0:42:57dangers to be around this kind of radiation
0:42:59i'm gonna turn on a detector for the radiation something you've probably heard of geiger
0:43:03counters you see to use
0:43:04all the time on the television when you're talking about radiation like turn this on
0:43:10when it detects photons
0:43:12of gamma radiation italy
0:43:14and you can hear
0:43:16bleeping right now
0:43:18the reason is there's cosmic radiation around this gamma waves are hitting us all the
0:43:22time
0:43:23and the little packets
0:43:25are detectable by this device
0:43:27now
0:43:28there's a lot of them always this is that right
0:43:31gamma source
0:43:32so this is intense
0:43:34radiation compared to the background radiation
0:43:37and lead to bring that
0:43:38in front of the detector
0:43:42and now watch i'm gonna bring the
0:43:43source closer and closer
0:43:46and listen
0:43:47you can see how bright this is
0:43:51there you know ten or fifteen photons every few seconds and now
0:43:59really getting almost continuous bombardment at all times already so fast brighter and brighter fish
0:44:05more intense as you bring the source closer
0:44:10to the counter
0:44:11each one of those little blips and individual photon individual particle and individual packet of
0:44:18light
0:44:21so we've learned that
0:44:22light has a particle nature
0:44:24there's a way of property associated with light and electromagnetic radiation and also a particle
0:44:29nature packets of energy being carried along the way
0:44:33a brilliant experiment the demonstrates that property is the photo electric affect
0:44:37and here's how work to take a piece of metal
0:44:39now model is an array of metal atoms
0:44:43and each of those atoms phones on rather loosely to its outer a lecture
0:44:48that's why the metal conducts electricity those electrons are rather free to move about the
0:44:52surface
0:44:53now if you shine the light on that surface
0:44:57what happens
0:44:58well you can shine a light of star in wavelength you will bring in a
0:45:01rather long wave like a red
0:45:03photon
0:45:05and when a rat beam of light
0:45:07hits that
0:45:08you'll find for many medals
0:45:11nothing happened
0:45:13and even if you make the like very bright
0:45:15very intense
0:45:17nothing happens
0:45:19if you bring in a greedy
0:45:21beam of light
0:45:23this now has higher energy photons shorter wavelength higher energy
0:45:29what you fine is the electrons are indeed injected
0:45:32it says photons are striking electrons and taking them off the metal
0:45:39you bring in a more intense a green light
0:45:42and you get more electrons they don't what way faster you just get more electrons
0:45:47with a brighter light
0:45:48all with the same kinetic energy
0:45:51you bring in
0:45:53blue light now higher energy photons even a shorter wavelength
0:45:57and you find that electrons with even higher kinetic energy a rejected
0:46:01and again the same correlation with brightness if you make the like brighter
0:46:06you get more electrons per second release
0:46:09so it's as electrons are caught in a well
0:46:13and there is an energy barrier that so the electrons
0:46:17next to the mat
0:46:19you have to overcome that energy barrier to reject the lecture on and release it
0:46:24from the middle
0:46:25well
0:46:25designate this
0:46:27threshold energy or this
0:46:28well that
0:46:30with the symbols p
0:46:32so
0:46:33red light
0:46:35the four times of read like don't have enough energy to even get out of
0:46:39the well
0:46:40so it doesn't matter if there is more of them if there's more intense light
0:46:43more photons per second
0:46:46still not of the electrons
0:46:48leave the well
0:46:50electrons of
0:46:51our photons in the green region
0:46:54so shorter wave like have enough energy
0:46:57to overcome this
0:46:59binding energy of the electron being held of the metal
0:47:02and
0:47:03a little bit of kinetic energy
0:47:05more energy still
0:47:07in blue photons
0:47:08in jack's
0:47:09electrons with even more kinetic energy
0:47:12so it says
0:47:15the
0:47:16photons of light
0:47:17are coming in and jostling electrons like i'm holding onto this tennis ball photons are
0:47:23coming in and jostling them
0:47:26the higher energy the photon
0:47:29the more jostling occurs
0:47:31until
0:47:33you get to a photon they can actually
0:47:36higher energy now
0:47:37brink
0:47:38no photon three
0:47:40of the metal
0:47:42can i might as well but
0:47:46if you go to higher and higher energies
0:47:50bigger and bigger photons in this case blue light
0:47:54you inject the electron with more kinetic energy
0:47:57no brightness doesn't matter we said well brightness adjust more photons per second that just
0:48:02paper i backup
0:48:04but not enough energy one photon
0:48:07per lecture on to reject any single electron
0:48:11but
0:48:11binning photons high energy blue lights a
0:48:15comes in and slams that metal
0:48:18and really stands the electro offline
0:48:21sorry so a high energy
0:48:25is what we have
0:48:26so we can actually plot
0:48:28we can plot the kinetic energy of the electron
0:48:31versus the frequency of the light that we shine on the middle
0:48:35and we know
0:48:36up to a certain frequency no electrons rejected
0:48:40and then you reach that threshold will just get that you'll just overcome the binding
0:48:45energy holding electron to the metal and you start to a job electrons then higher
0:48:50frequency like
0:48:51just gives you more kinetic energy in the electro
0:48:55the energy the phone time we know is age a new
0:48:59so we can write the kinetic energy that the electron has
0:49:03is the energy that the photon comes in that minus this binding energy
0:49:08so only access energy of the photon goes into kinetic energy
0:49:13so you can write the both energies in terms of photons and you can realise
0:49:16there's a minimum photon energy required
0:49:19do we jack electron from the map
0:49:22if you look at different metals
0:49:25different males have different threshold frequencies
0:49:28prince and you could have a metal that's described by a blue photon is the
0:49:32minimum
0:49:33photon that injects an electron
0:49:35and higher energy photons
0:49:37you
0:49:38electrons with more kinetic energy
0:49:41so this problem the for electric affect
0:49:44helps us understand the particle nature of like
0:49:46and it's actually albert einstein
0:49:49i gini assume that this problem
0:49:51so i just before the for the electric affect was understood in the particle nature
0:49:55of light was understood were taken
0:49:58their medals shining lights and increasing the intensity figuring
0:50:03actually not cost more electrons
0:50:05but increasing intensity very bright light
0:50:08didn't do anything
0:50:10and when you could reject electron
0:50:12increasing the intensity
0:50:14didn't increasing energy of electrons you just got more electrons coming off with the same
0:50:19energy
0:50:20well
0:50:21it takes aging is often to look at a
0:50:23a very troubling problem and see it in a whole new like
0:50:27and that's what einstein did he said what that looks like
0:50:29the lights behaving like particles
0:50:31it looks like little bit some later coming in so bright light is just lots
0:50:35of bits
0:50:37but they all have the same energy
0:50:38so those lots of bits inject lots of electrons each electron with the same energy
0:50:44so the for electrical fact and albert einstein have helped us understand
0:50:49the particle nature of like
0:50:57we're in the demo that with money we set up a photo electric affect experiment
0:51:01e as white light
0:51:02impinge and on the metal
0:51:03in the energy of the photo electrons admitted
0:51:06are shown on the meter it right
0:51:08if you put a red filter
0:51:10in the path
0:51:11now only read of photons can strike the metal and you can see they do
0:51:16not have sufficient energy to eject photo electrons
0:51:20if we go to higher energy photons say
0:51:23go to yellow lighted that's shorter wavelength and higher energy photons
0:51:29they may be able to eject photo electrons
0:51:34and indeed for electrons are omitted
0:51:36but with a relatively small energy
0:51:41if we go to a higher still
0:51:43photon energy green wave links
0:51:47that's shorter wavelengths still higher photon energy use
0:51:51we can see
0:51:53full electrons emitted at a higher energy
0:51:58now we can still go to higher energy photons
0:52:00blue
0:52:01photons in the blue region
0:52:04are the highest energy visible photons and honey has a blue filter that will pass
0:52:09only blue photon
0:52:12putting a blue filter in the path
0:52:15shows photo electrons emitted at the highest energy
0:52:19so here's an example of the photo electrical effect
0:52:26thank slightly for that great demonstration
0:52:30let's talk about the full electrical fact in terms of it can quiz
0:52:33if we shined a beam of light on a certain metal it has no effect
0:52:38the question i have for you is what change in that be should i make
0:52:43in my best hope to eject electrons
0:52:45should i increase the intensity of the light
0:52:48the increase the
0:52:49wavelength of the light
0:52:50or
0:52:51increase the energy
0:52:53of the like
0:52:54think about that per minute and make a selection
0:53:10concerning is
0:53:11possible arguments for each of the answers
0:53:14survey
0:53:14increased intensity means morpho time strike electrons increasing electron kinetic energy
0:53:22b
0:53:22decrease in the way like increases the photon energy which increases the kinetic energy imparted
0:53:28the electro
0:53:29or c
0:53:30increase in the energy of the photons increases the kinetic energy imparted to be electrons
0:53:36consider those arguments
0:53:37and make your selection again
0:53:50when you think about the photo-electric affect you think about the
0:53:54energy the wavelength the frequency all those are related
0:53:59of the photon striking the metal
0:54:01if you have a photon that won't inject electron what you need
0:54:05you need photons with more energy you have to increase the energy the photon
0:54:10increasing the number of photons won't do
0:54:13you'll just track a lot of electrons with a little bit of energy that doesn't
0:54:16help you
0:54:17so increasing the intensity is not gonna work
0:54:20you want more energy in each photon
0:54:22the energy photons is age new or h c overland that so it inversely proportional
0:54:29to the wavelength
0:54:29directly proportional to the frequency
0:54:32so if you want to increase the energy you should decrease the wavelength
0:54:38so
0:54:39increased energy photons will work
0:54:41answers at but
0:54:43if you are clever you also noticed
0:54:45i'd decreased
0:54:48wavelength in the photon will also work because that will increase the energy
0:54:52so here you could have answered b or c
0:54:56and you would have got an electron ejected from that now
0:55:00let's look at the for electric affect again
0:55:03which combination of a photon striking a metal
0:55:07it jack select run with the highest kinetic energy so i have to nettles represented
0:55:11and several different
0:55:13photon energies
0:55:14so is of the yellow photon striking middle one of green photons tracking metal one
0:55:19or a blue photon
0:55:21striking that'll number to think about that for a second and make a selection
0:55:38let's consider an argument for each of the three answers
0:55:41yellow light
0:55:43is high energy
0:55:44with a lower threshold metal
0:55:46so it'll give the highest kinetic energy electro
0:55:48or b
0:55:49green light is higher energy than yellow light and the striking the same l
0:55:54blue is tracking a higher threshold metal
0:55:56so green under the one should be dies or
0:56:00blue light is the highest energy of all three folder
0:56:03so which reject electrons with the highest kinetic energy
0:56:07think about those three for a minute and make another selection
0:56:22so which combination of photon in metal gives you electron with the highest kinetic energy
0:56:27let's look at all three
0:56:29yellow light striking metal one well i kind of outline
0:56:34about where
0:56:35the frequencies are of course a single frequency can't
0:56:39encompass the whole band of green in the whole band of blues
0:56:43so
0:56:44the largest yellow is somewhere below the green that's all we know
0:56:47so it somewhere in here
0:56:49so the largest possible in yellow photon
0:56:52striking that'll one
0:56:53we give a kinetic energy here
0:56:56what about green striking metal one
0:56:59but we know the green photon will be higher energy
0:57:03then the yellow photon
0:57:05striking metal one will give a higher kinetic energy
0:57:09what about blues photons tracking metal to well here's the blue region
0:57:13so somewhere in here will have a photon
0:57:16but no strike this metal with a high here
0:57:18threshold energy
0:57:20so you can drink the kinetic energy right off the plot
0:57:23so a blue photon even though it's the highest energy is tracking a higher threshold
0:57:27metal
0:57:28resulting in low were kinetic energy portal electron
0:57:32so
0:57:32green
0:57:33light on metal one will give the highest energy
0:57:37electrons injected from this metal system
0:57:41light
0:57:42has both the properties of a wave
0:57:45and the properties of a particle
0:57:47it's a way even a particle at the same time there's a duality to the
0:57:50way particle relationship
0:57:53what properties like wavelength
0:57:56and frequency and speed
0:57:59somehow must be related to particle property
0:58:02particle properties that are moving
0:58:04the most important property is the momentum
0:58:07it's mass times its velocity
0:58:09so how do we reconcile the two
0:58:11well
0:58:12light
0:58:13photons of light
0:58:14carry no mass
0:58:16so how can i carry momentum
0:58:18well they carry relativistic moment them
0:58:20their energy is given by h new or h c overland
0:58:26the energy can also be expressed
0:58:28by the and c squared
0:58:30relativistic energy
0:58:32well we can cast that in terms of the relativistic momentum
0:58:37so a single quantity and this p
0:58:40so using those two relationships we can derive that the momentum
0:58:45he's
0:58:46planks constant divided by the wavelength
0:58:49so here's a single relationship
0:58:52that shows
0:58:54particle and wave nature
0:58:56in the same relationship
0:58:58so particles and waves a duality
0:59:02the wavelength is related to the momentum the momentum is related to the wavelength
0:59:07for light
0:59:08bunks constant
0:59:10unites the two
0:59:11six point two six two times ten to the minus thirty four jewel seconds
0:59:16way
0:59:17particle
0:59:18acting together
0:59:19sometimes the properties of the particle
0:59:22exert themselves
0:59:22sometimes the properties of the wave exert themselves
0:59:25they both exist together all the time particle and wave nature of like
0:59:33we've talked about four times as the smallest particle of light
0:59:37and we said when you get down to the photon level if you split that
0:59:42photon further it loses the properties of that like
0:59:45and indeed
0:59:47splitting the balloon photon
0:59:49in all their photons is possible
0:59:51but it no longer has the properties of the balloon like that originally had it's
0:59:55the smallest particle of blue light
0:59:57but you could have
0:59:58but splitting it is still possible
1:00:00so let's talk about that
1:00:01a photonic four hundred centimetres
1:00:03is split into two
1:00:05one of the photons the comes out is that well on an animator
1:00:09what is the wavelength of the other photon
1:00:12so
1:00:12let's consider that is it at
1:00:14two hundred animators be six hundred or seen eight hundred animators
1:00:19what is
1:00:20that other photon think about that for a minute
1:00:23and make your selection
1:00:36let's consider an argument for each of the answers
1:00:39hey
1:00:39splitting a photon must reduce the wavelet
1:00:42and two hundred is the only smaller waves
1:00:44b
1:00:45energy is conserved and it is inversely proportional to wave like
1:00:49so one over four hundred
1:00:51equals one over six hundred plus one over twelve
1:00:54or see the sum of the wavelength
1:00:56must be cancer and add to the long longest we've like
1:00:59so
1:01:00twelve hundred is four hundred plus eight hundred
1:01:04consider those three arguments and make a selection again
1:01:18when we split a photon
1:01:20in to other photons
1:01:23the total energy that we start with can be lost
1:01:26so energy will be cancer
1:01:28so the two smaller photon energies
1:01:30must add to the original photon energy
1:01:33and photon energy at as the inverse
1:01:36of the wavelength
1:01:37so in d
1:01:39h t over four hundred
1:01:42most equal itsy over twelve hundred plus the other photon emitted and that must be
1:01:48h t over six hundred
1:01:50so in d
1:01:51to lower energy photons add to give the high energy photon and the correct answer
1:01:56here is b
1:01:58a six hundred animator photon
1:02:00is split along with a twelve hundred animator photon when you split that four hundred
1:02:04and a beautiful
1:02:07let's look calculation involving the full electric affect
1:02:11injected for all electrons and protons of a certain energy getting a map
1:02:15we'll take the question
1:02:17what we have like the radiation must use to eject electrons
1:02:21with the velocity
1:02:23given from chromium that'll with a work function that's given
1:02:27so this work function for chromium metal four point three seven electron volts
1:02:31is how strongly chromium hold onto it electron
1:02:36so we have to say well what is the photo-electric affect
1:02:39the for electric affect we have to balance the energies
1:02:42remember the photo lexicon fact has
1:02:45the
1:02:45kinetic energy of the electron
1:02:48is
1:02:48the
1:02:50photon energy minus
1:02:52the work function
1:02:53for the men
1:02:55so
1:02:55but not a holds onto the electron
1:02:58but if we bring in a high enough energy photon
1:03:01we can eject an electron with a certain kinetic energy
1:03:04so that so we have the fine
1:03:06let's look at what wave like will work for this problem
1:03:09and what will do is will balance the energy is involved
1:03:13with a common unit
1:03:14and the units we always use r
1:03:17kilograms
1:03:18meters seconds and jules
1:03:20yes i units
1:03:22now we do that to keep
1:03:24all the things we multiply together
1:03:26consistent unit wise
1:03:28where dimension really consistent
1:03:30we always use kilograms for mass
1:03:32so don't just put your masses into your equations with
1:03:35grams or pounds or some random asked unit
1:03:37you come across the mask converted to get would right
1:03:40we come across the distance a like
1:03:43convert that two meters
1:03:44at time seconds
1:03:46and energy
1:03:47you
1:03:49that would keep your energies can your units consistent and allow you to do the
1:03:52calculations correct
1:03:54so again the kinetic energy of the electron is
1:03:57the
1:03:58energy of the photon minus the work function of the metal
1:04:01we can calculate the kinetic energy of electron we're gonna have to look up its
1:04:05mass
1:04:06and use the velocity that we've been given
1:04:09so
1:04:10well use
1:04:10kilograms for its mass
1:04:13meters
1:04:13for the link per second
1:04:15for the velocity
1:04:17meters per second square
1:04:19mass
1:04:19velocity squared
1:04:21that's gonna be an energy a kinetic energy so this will be jules
1:04:25in fact that's how remember the s i units of jewels
1:04:28i remember kinetic energy is a jewel
1:04:30and
1:04:31kinetic energy is mass times velocity squared
1:04:34so it's kilogram meters square per second squared
1:04:38so this is then pretty straightforward will look up the
1:04:41mass someone electron express it in kilograms
1:04:44express are velocity was given in kilometres per second but i'm gonna converted to meters
1:04:49per second
1:04:50to give a units the same
1:04:52square that that's relatively straightforward eleven point sixteen
1:04:56jules time standard the minus ninety
1:05:00tidy number of jewels course of the water
1:05:04so
1:05:05with that kinetic energy
1:05:07we can continue to balance our energy we know the kinetic energy
1:05:10is eleven point sixteen times ten to the mine is nineteen jules
1:05:14the work function we've been given
1:05:17that's four point three seven electron volts
1:05:20electron volts of a unit of energy
1:05:22it's
1:05:23we kinetic energy than an electron gains
1:05:25as you accelerated across a potential of one vote
1:05:30we don't have to know that but it is nice to know that
1:05:33i can convert jules to electron volts with a simple conversion factor i can look
1:05:37up in any technical
1:05:39one point six one times ten to the mighty minus nineteen jules product rumble
1:05:46so
1:05:46we can do that product so the work function in terms of jules
1:05:51seven point o four times then the minus nineteen you
1:05:55so
1:05:55well rearrange now and saw for the energy of the photo the energy the photon
1:06:00is the kinetic energy of the electron plus the word function
1:06:03that has t will eleven point sixteen times ten the might is nineteen jules
1:06:08and
1:06:08we're gonna at non of that work function seven point o four times ten the
1:06:13might is nineteen tools
1:06:15we can express the photon energy in terms of its wavelength
1:06:18as well as its frequency
1:06:20and that has to go to some of these two or
1:06:22eighteen point twenty can stand for the minus nineteen jules
1:06:27of course
1:06:28we can solve a the wavelength now that's just h c over eighteen
1:06:33point twenty times ten the midas nineteen jewels and we can do that matter
1:06:37this number plug in
1:06:40blogs constant and the speed of light in jules per second and meters per second
1:06:45and then we'll notice that the units of jules will cancel out
1:06:48and the units of seconds will cancel out and levers units of meters
1:06:52and that we want always good to check
1:06:55due to units that we have laughed
1:06:57makes sense for the quantity that we're solving for
1:07:00we're solving for wave link a link
1:07:02do i have meters
1:07:04in this case i do
1:07:06so
1:07:06i have
1:07:07doing them at hunter nine times ten the minus nineteen
1:07:11meters
1:07:12should be hundred
1:07:13nine times ten the might is nine meters at an animator
1:07:17under nine animators
1:07:20is as we recall
1:07:21in the u v the ultraviolet region
1:07:24we know visible went from seven hundred down to four hundred millimetres
1:07:28well all four hundred animators at higher energy
1:07:31are the ultraviolet photons
1:07:34so an ultraviolet photon
1:07:36is required to reject
1:07:38an electron
1:07:39from chromium at all
1:07:42so we understand light
1:07:44as behaving like a way is like an ocean
1:07:46with a wavelength and sp but also like a particle a little packet of energy
1:07:52and the energy amount is each times new punks constant times the frequency of the
1:07:57like
1:07:58now
1:07:58a particle
1:08:00has momentum
1:08:01and we've seen the momentum of the particle that photon
1:08:06can cause an electron to be rejected from the metal
1:08:09we saw in the photo like to contact
1:08:12a
1:08:13incoming photon injecting electrons from a metal
1:08:17so
1:08:18how does that particle nature and wave nature reconcile themselves
1:08:24well let's talk about that
1:08:26the
1:08:26light
1:08:28way particle duality
1:08:30you have
1:08:31like that is like a wave we understand with a wavelength
1:08:35and of frequency and this me
1:08:38we can write down it's
1:08:40energy as a particle
1:08:42as h times new
1:08:43and
1:08:44each time see over lambda
1:08:47using the waves properties to write the energy in two different ways
1:08:51now the
1:08:53wave particle the light particle that we call a photon
1:08:58has a momentum we've seen it can transfer momentum
1:09:01from the photon to the electron
1:09:03but the momentum
1:09:06we often associate with mass
1:09:08but
1:09:09the photon has no mass
1:09:11the photon is a particle
1:09:13and it
1:09:14mass less moving at the speed of light
1:09:17but we can say the energy of the particle use
1:09:20einstein's
1:09:22equation for relativistic
1:09:24particles moving here the speed of light
1:09:27and c squared is at the energy
1:09:30now we have to expressions for the energy the energy of the photon and the
1:09:34relativistic energy and c squared
1:09:38mass
1:09:39times velocity
1:09:42mass times the speed of light c in this case for a photon
1:09:45can be written as
1:09:46the moment sometimes seen so
1:09:48the momentum is
1:09:50and times c
1:09:52times another cu gives you and see square
1:09:54so these expressions for the energy
1:09:57a pull into these expressions to the energy so what we can do is write
1:10:01the momentum then
1:10:03in terms of these two energies and we'll find the momentum
1:10:06is
1:10:07the
1:10:08points constant divided by the wavelet
1:10:11so
1:10:11a simple relationship
1:10:13but wean
1:10:14the momentum and the wavelength
1:10:16the particle characteristic momentum
1:10:19and the wave characteristic the wavelength
1:10:21both expressed at the same time waves and particles
1:10:26acting
1:10:27the way they choose
1:10:28sometimes like will be like a way sometimes a behaves like a particle
1:10:33there's a duality between them
1:10:35expressed by this beautiful relationship between the wavelength
1:10:39and the momentum
1:10:40bunks constant again extremely small number is the proportionality constant between the momentum and
1:10:47the wavelength
1:10:49so waves
1:10:51particles
1:10:52light
1:10:53we have to think of them all at the same time
1:10:57light
1:10:58behaves like a way
1:10:59and a particle
1:11:01and we came up with a beautiful relationship between the particle nature its momentum and
1:11:06the wave nature of the wavelength
1:11:09it be interesting to say let's go back to particle
1:11:12could be possible that particles have wave nature
1:11:16we were little surprising we saw
1:11:17the wave like property light
1:11:20have a particle nature
1:11:22is the dichotomy gonna continue
1:11:24is eight particle
1:11:27able to have a wave like property
1:11:28well we would define it in the same kind of way we would say a
1:11:31particle has a momentum
1:11:33and electrons for instance they can move
1:11:36and they'll be moving at a certain velocity you take the product in their mass
1:11:40in the last me that some momentum
1:11:43way of would have a weighting the like
1:11:46if we say well we had this relationship that had
1:11:50wavelength and momentum in it
1:11:53to relate
1:11:54for light
1:11:56the duality between the particle wave nature
1:11:59the same thing can be applied to matter
1:12:01and this is actually very fascinating concept how can a particle like an electron
1:12:07also behave like a way
1:12:09i think you can kind of by the light argument like behaves like a weight
1:12:14and we saw with the fraction interference
1:12:16we had one with going like this and one-way point like this
1:12:20when they came together and they added constructively you gotta bright spot
1:12:25when they added these productively you got a dark spot
1:12:29and that was very wave like
1:12:31we saw their particle like nature of light in the photo-electric affect
1:12:36particles
1:12:37from matter that's obvious if there is a particle of matter
1:12:42there's particles of matter atoms electrons fundamental particles
1:12:47but will they also have a wavelength will there be did duality here
1:12:51it turns out there is it's one the most fascinating properties of nature that very
1:12:56tiny particles
1:12:58have
1:12:59a wavelength associated with them
1:13:01we associate the wave like to the same relationship we used for light
1:13:05we call this that the role be relationship the momentum and the wavelength related
1:13:10for
1:13:11particles
1:13:12we're gonna use the intensity of their way
1:13:16squared
1:13:17as the probability of the particle being found in that region
1:13:22we'll talk about
1:13:23squaring the wave function of a matter
1:13:26to get the probability whether that matter actually exists there
1:13:30so waves and particles
1:13:32two
1:13:33totally the similar things but
1:13:36the
1:13:37particle and wave the welding works for matter
1:13:40so
1:13:41how can we prove
1:13:42well remember how we prove that like behave like a way
1:13:46we
1:13:46shined the wave through some obstacles we got this the fraction interference pattern
1:13:51bright spots and light spots on the scree that's it'll but that's a wave like
1:13:55property
1:13:57matter we understand as a particle already
1:14:00can we just demonstrate its wave like nature
1:14:03what turns out you can and you can take electrons
1:14:07and you can shine electrons at little obstacles in this case you use the spacing
1:14:12between the layers in the crystal
1:14:15you shine an electron at that
1:14:17and that the
1:14:19gives you the fraction in interference
1:14:21and if you shine that on the screen like you did with the
1:14:24the light
1:14:25you find that there's places on the screen that the electrons hit and you get
1:14:30bright spots lots of electron sitting in certain spots and places on the screen with
1:14:35a light does not
1:14:38the
1:14:39the electrons
1:14:40passed through their grading their crystal there it's very likely to hit itself some spot
1:14:46it's unlikely to zero probability that they'll hit other spot
1:14:51and that's very strange
1:14:53if you take just the b gun
1:14:54or machine then you should it through an obstacle there's no place a be on
1:15:00the slits where the
1:15:03well it's cannot yet
1:15:04there's no excluded particles there's always put it space where you cannot hit
1:15:09with electrons at a different story you passed through that rating you passed through that
1:15:15crystal
1:15:16and there are places where the electron is forbidden to hit
1:15:20there's electrons interfering with each other
1:15:24you're allowed to hear you're not allowed to hit here there's
1:15:28dark and light spots that
1:15:31detect the electrons
1:15:32high intensity of problem high intensity high probability finding electron in certain spots
1:15:37low probability dark spots in other spots on the screen
1:15:41electrons be anything is
1:15:43their interfering with each other
1:15:46just like they have a wave property
1:15:48fantastic property of electrons that we can actually demonstrate
1:15:52so it's easy to show
1:15:54electron
1:15:56give you a probability distribution
1:15:58you'll have
1:15:59electrons
1:15:59going through their crystal braiding
1:16:03likely is very strongly to hit it some spots on likely to hit other spot
1:16:08so
1:16:09matter
1:16:11electrons in particular this playing
1:16:13wave like proper
1:16:19particles
1:16:20can behave like waves
1:16:21they can have a way like property
1:16:23and the way like property wavelength is given by the broadly rate relationship a particle
1:16:28the has a momentum
1:16:30amassed and a velocity will have a wavelength given by the momentum
1:16:35planks constant divided by that we've like
1:16:37let's look at some concrete examples
1:16:39to see actual particles and what they're the probably with like would be
1:16:43here's a list of particles and their to probably wavelength of proton
1:16:47now that's a particle of light
1:16:50that obviously has a wave like that we've talked about a yellow photon it's the
1:16:54probably wavelength six hundred nanometres
1:16:56how about an electron moving at end of the fit meters per second
1:17:00so
1:17:01electron just zipping along in space
1:17:03you can use of the probably relationship
1:17:05the find a
1:17:07the probably wavelength
1:17:09of around six centimetres for that electron
1:17:12a sony an atom
1:17:14at
1:17:15eight hundred
1:17:16or at calvin
1:17:18that's a temperature
1:17:19that if the time determines the average speed in the system
1:17:23the average speed of those particles around three hundred meters per second
1:17:28we know they're sodium adam so we know their mass
1:17:31but momentum i can calculate a wavelength
1:17:33a few hundreds of an annotator
1:17:36now let's take a baseball and of an object that we know the size and
1:17:41mass out on a macroscopic
1:17:43object
1:17:44a macroscopic object like a baseball a hundred and seventy grams set
1:17:48by the
1:17:50major league baseball association
1:17:52a standard baseball rolled at forty meters per second
1:17:56that's a very good fast ball
1:17:58we can calculate using an appropriate relationship the wavelength
1:18:02but look at how small the number
1:18:05this is tend to the minus twenty six an animator we're already at an animator
1:18:09stand of the mind is not
1:18:11now we've gone
1:18:12that tend to the minus twenty six of the rules
1:18:15this is incredibly small distance
1:18:18it is so small it is insignificant
1:18:21and that's what you say if you take a macroscopic objects by take is
1:18:25tennis ball and i throw it i get a velocity
1:18:28i don't notice a wavelength
1:18:31and you don't notice the with like because the way like there's vanish really small
1:18:35in order for the wave like properties of matter to manifest itself the matter must
1:18:40be very tiny
1:18:42if you have very tiny matter with very tiny moment
1:18:46then
1:18:47the wavelength
1:18:49creeps up into a region where you could actually detected
1:18:52so
1:18:53particle and wave nature of matter is gonna be important for small particles but not
1:18:59for macroscopic large particles we don't even notice
1:19:05particles
1:19:06all electrons
1:19:07small atoms have we like properties let's look at that in terms of it can
1:19:12quit
1:19:13how many photons
1:19:14they're gonna behave like particles should about its a
1:19:18that's only an atom
1:19:20at calvin
1:19:22the
1:19:23sodium adam
1:19:25impacted by
1:19:26the wavelength of light behaving like a particle behaving like a photon
1:19:30about how many will start that
1:19:32about one about a hundred about ten thousand
1:19:35think about that and make your selection
1:19:51let's look at possible explanations for each answer
1:19:54a one particle
1:19:56well interactive one-way by that are probably relationship so to be a one-to-one relationship
1:20:01b
1:20:02a hundred photons reduce the temperature about a hundred k
1:20:06that's near zero and the sodium adam should be about start
1:20:10or c
1:20:11this only madam wavelength
1:20:13is about one ten thousand the photon wavelength so sent ten thousand four times are
1:20:19needed for you will transfer of moment
1:20:21think about those three explanations and make a selection
1:20:38matter has both wave and particle characteristics
1:20:41we seen the
1:20:42momentum and the wavelength
1:20:44are related by the to probably relationship
1:20:46and we calculated it for several
1:20:48different objects
1:20:50now we're saying
1:20:51well as only a madam
1:20:53is gonna be travelling at
1:20:55about three hundred meters per second it's gonna encounter photons of six hundred
1:21:00then a meter waveline
1:21:02about how many photons have to strike those only am atoms to get them to
1:21:06slow down
1:21:06in about stuff
1:21:08well if you look at this
1:21:10here's the sodium adam
1:21:12travelling at
1:21:14at k that's its temperature at about three hundred meters per second in has a
1:21:19wavelength as we've seen abouts extensive in and are six hundred seven and you're
1:21:22those
1:21:23photons
1:21:24of yellow light coming in at six hundred millimetres we can say well i want
1:21:28the moment
1:21:30of these two systems to be equal i wanted transfer enough momentum from these waves
1:21:36two
1:21:37stop
1:21:37the particle
1:21:39so
1:21:39the way as a particle nature
1:21:43it has a tiny little momentum
1:21:45i need to transfer it to this larger momentum your consists only mammon i one
1:21:49that
1:21:52keeping with four times and tell
1:21:57that's only in atom about stops
1:22:00and i can do that i just calculate the moment of each and as i
1:22:03do so i see if the moment are related by the wavelength by the role
1:22:08be relationship and we can see
1:22:10the momentum of
1:22:12the
1:22:14photons
1:22:15are about ten thousand times smaller than
1:22:18the moment of
1:22:19this only a matter
1:22:21so what i near about ten thousand
1:22:24four times to about star consortium
1:22:27you can actually do this experiment it's
1:22:30that i do this
1:22:31the
1:22:31this
1:22:32experimenter to do this
1:22:33were awarded the nobel prize
1:22:35for what's called laser schooling
1:22:37you can take
1:22:39atoms and slow them down the smallest lowest temperatures ever achieved
1:22:45by calling things by traditional means
1:22:48in refrigerators and by vacuum pumping
1:22:50and then using this additional method using lasers
1:22:54the trap the atoms and bonds photons off them to leave virtually
1:22:59come to a stuff lowest temperatures ever achieved by what's called
1:23:04laser cool
1:23:05the correct answer for are laser cooing experiment is about ten thousand yellow photons
1:23:10the stuff that's only that
1:23:17we're in the demo that with honey i best in to give us a demonstration
1:23:20of quantisation of waves
1:23:24see what you can come up with
1:23:31he's got to tennessee spins it we get a total
1:23:37i are
1:23:41here
1:23:45all
1:23:48so five distinct tones rather than a continuous
1:23:52mm
1:23:54so we see quantisation of the with links that it in the length of that
1:23:59too
1:24:00quantisation of acoustic waves
1:24:04particles especially tiny ones have a way like property that we can result
1:24:10and we saw that with electrons going through a crystal
1:24:13the for acting
1:24:15some particles
1:24:16of the wave hitting it some portions of the screen
1:24:19somehow any other portions that screen
1:24:22some portions of the screen forbidden to hit
1:24:24it's if the electrons
1:24:26come through
1:24:28interfere with each other like waves and are low only allowed to hit certain
1:24:32points on the screen
1:24:35i think it's a little spoke you than that
1:24:37if you send the electrons through this experiment
1:24:40one electron at a time
1:24:43base still
1:24:45can not yet certain parts of the screen
1:24:47i think you can imagine all electrons a lot of them behaving like waves and
1:24:50interfering with each other
1:24:53but the electrons behaving like both particle and wave when they go through that grading
1:24:58they somehow no
1:25:00not to hit certain parts of the screen
1:25:03it's a very
1:25:05spooky property of matter
1:25:07and from here on in chemistry we're gonna talk about those various key quantum properties
1:25:12of matter
1:25:13let's start by looking at a classics experiment or classic calculation of particle in the
1:25:20box you can imagine taking a particle that has a wave like property and trapping
1:25:25it
1:25:25in a small region of space
1:25:28when you trapped in a small region of space
1:25:30you get we like properties occurring
1:25:33and those with like properties are well defined when you trap
1:25:38the particle
1:25:40now
1:25:41you trap the particle in space let's drop picture of that
1:25:45a wave like
1:25:47particle
1:25:48trapped in space i'll draw like yes all say
1:25:51blocks of length l
1:25:53and the particle has to me between here and here
1:25:57it'll have a way like property so i'll draw a wave like expression
1:26:02on this box
1:26:04now
1:26:05this way like expression i'll call the weighting function of that particle
1:26:10the wave function of that particle
1:26:12will you have the designation side
1:26:14in this case i of x because this is the x dimension in space
1:26:19now we've already said that the intensity of the wave squared
1:26:23is going to give us
1:26:26indication of the probability of finding the particle
1:26:29so
1:26:29the probability
1:26:30size where gives us the probability of finding the particle
1:26:33clearly in this case the probability flying in the particle rate the middle of the
1:26:37box
1:26:38would be very high if this particle behaves like a way
1:26:42well how do i come up with these way functions
1:26:45well i can actually do some that max i can say
1:26:48write down the physics
1:26:51the kinetic in the potential energy of the particle
1:26:54and the boundaries that and putting on the particle
1:26:58and when i do those things
1:26:59i can solve the differential equation
1:27:02for the particle
1:27:04stuck in this tiny little box
1:27:06it's behaving like a wave their those wave functions i
1:27:10these are how the wave functions i must interact with each other
1:27:13if it's a particle to be stuck in this box
1:27:16now
1:27:17we will go through all the mathematics but you can solve this
1:27:20and when you solve this equation you get an expression for side it's not that
1:27:24harder to heart understand and it looks
1:27:27like a weighting function is a sign function of oscillating
1:27:30sine wave
1:27:31that you recognize junior high school mathematics science axe
1:27:35is sign of x
1:27:37the length of the box
1:27:38is in there and another parameter and is in there there's not just one solution
1:27:43to this expression there's multiple solutions
1:27:47on the integers and
1:27:50work
1:27:51and give you a solution to this equation
1:27:53so that means there's not one-way function that works there are several wave functions that
1:27:57work
1:27:58you can have a ground state the smallest value of and
1:28:02but you can have what we have excite what we call excited state higher values
1:28:06of and
1:28:08also give weight functions and you can tell what happens this value and is here
1:28:12in the sign expression is that integer and gets larger you get more
1:28:18waves appearing in your box
1:28:21so these functions
1:28:23so i
1:28:25give us the probability when we square them of finding the particle at various locations
1:28:30in the bar
1:28:31we can also say well what's the energy of that article
1:28:34we can calculate the energy using our expression for the wave function in the probability
1:28:39square
1:28:40and we can say well i'll ranking the energy these particles increase in energy as
1:28:46n increases
1:28:49the expression for the energy of the particle
1:28:51in terms of this
1:28:53quantum number we're gonna call it now and
1:28:58go like
1:28:59the energy of the and state and square h where
1:29:03over eight and elsewhere the length of the box the quantum number punks constant are
1:29:08all in there
1:29:10so i can then plot out well the lowest
1:29:13energy state
1:29:14and equal one we have no an equal zero state any one is where we
1:29:19start our county
1:29:21and equal to higher energy state
1:29:23you'll see and goes in as the square
1:29:27so
1:29:27higher and higher energy state
1:29:29any will three
1:29:32well also caught calculate and catalogue what we call the no
1:29:36the areas of the box where there's zero probability
1:29:41of finding a particle
1:29:43zero probability are the places where the weight function goes to zero
1:29:47after you square zero you still get zero
1:29:49so the probability of finding the particle at these
1:29:53crossings
1:29:54is zero
1:29:55and that's a strange characteristic
1:29:58of particles that behave like way there's
1:30:01portions of the box
1:30:02where the particle is forbidden to be
1:30:05so this is interesting
1:30:07i'll have to nodes one two
1:30:10note a area where the wave function goes to zero in the high energy state
1:30:15one note here and zero nodes here
1:30:18when the number of nodes increase the energy state increases that's a higher energy situation
1:30:25so here's what i have i have a way
1:30:29i'm bounding it
1:30:32on either side
1:30:34and like is on the demonstration when you have a way like property
1:30:38you bounded on either side only certain wavelength can exist
1:30:42so
1:30:43half a wavelength one full wave like
1:30:46one and a half wavelength
1:30:48i can't
1:30:49one of the third wavelengths in this box
1:30:51or a quarter of a wavelength because though
1:30:54particle has to go to zero probability at the ends
1:30:58so five
1:31:00the ends of the box
1:31:01i put boundaries on a way
1:31:04i naturally get
1:31:05what i call quantisation
1:31:07not every wave can fit in these boxes only certain in waves can fit in
1:31:12these boxes and there's a gap
1:31:14i go from here
1:31:15any point one
1:31:17although we have to here and i skip all those energies in between
1:31:21i can only have this energy states in the box
1:31:23all this energy state for the box
1:31:25and no energy states in between energy is one time
1:31:29and wayne's naturally do this
1:31:31it's not unusual to see it we can demonstrated with a new ways
1:31:35audio
1:31:36sound
1:31:37is a way of like property and here's a two
1:31:39a fixed like
1:31:41so if i one way is to exist on this to only certain wavelengths will
1:31:46fit
1:31:47i'll be able to fit
1:31:48no full wave on this or wave on this away even have on this
1:31:54so only certain
1:31:56sounds
1:31:57will fit
1:31:58in this too
1:32:00as interesting characteristic i can demonstrate a couple of the sounds
1:32:04that fit in this too
1:32:05we won't here when we when we
1:32:07figure the sounds that fit in this too
1:32:09we will hear continuous sounds we all here who h
1:32:15in tenuous wavelength
1:32:17well have
1:32:19and
1:32:19do
1:32:20to individual wave likes that fit in this box
1:32:24that's actually demonstrate that i'll try to spin this and you guys can listen
1:32:29there's one way
1:32:34no way it's and a you're higher frequency lower
1:32:45way modeling
1:32:48and there's that low frequency
1:32:51long way
1:32:52to energy levels and you can see energy takes be more energy that high frequency
1:32:57in this
1:32:58low frequency on a regular higher
1:33:02i higher one
1:33:07that lpc wonderful acoustic example
1:33:10of ways and quantisation
1:33:13all you need to get quantisation is take away
1:33:17and
1:33:17force it
1:33:19to exist in a certain area of space
1:33:21you fix the ends of the way you get quantisation
1:33:25you take nothing else home from cam one
1:33:27take on the fact
1:33:29that when you take away and use
1:33:31stick boundaries on it
1:33:33you naturally get quantisation where whether it's a matter way than electron stuck in a
1:33:37box
1:33:38or the acoustic way
1:33:40existing in this little primitive instrument
1:33:44matter as way like properties
1:33:46and waves are bounded you get quantisation
1:33:50you get particle in of a
1:33:54if i have a particle that has way like properties and strapped in a box
1:33:59we understand that has different energy states
1:34:03and how do i make a transition between one energy state and the next energy
1:34:07state
1:34:08well i have solar energy and absorb amount of energy equal to the gap
1:34:13between
1:34:13those energy states
1:34:16so let me ask you
1:34:17which is true about the photon energy i could absorb the energy in terms of
1:34:21photo
1:34:23absorbing each transition shown so here's transition a and transition be
1:34:27so a particle of you know
1:34:31box like l and a particle same article
1:34:34bigger about
1:34:35no i haven't drawn in be to scale the question is which transition is bigger
1:34:40has a larger energy transition is a bigger than be are but are they about
1:34:45the same
1:34:46or
1:34:46is a smaller than b
1:34:49think about that for a minute and make a selection
1:35:06that's looking up
1:35:06possible explanation for each of the answers
1:35:09a smaller box size means a larger spacing of the energy level so a is
1:35:15gonna be bigger than b
1:35:16or
1:35:17for b
1:35:18twice the box like means double the energy so the transitions should be about equal
1:35:23or c
1:35:25and equal to for ling the box
1:35:28is equal to any people for it to l box
1:35:31but the ground state slower
1:35:33so
1:35:33to l
1:35:34we'll have be larger
1:35:37then a
1:35:38think about those
1:35:40and make another selection
1:35:53we're looking at identical particles trapped into different boxes one box twice the size of
1:35:58the other box
1:36:00so
1:36:01if you look at the particles trapped in the boxes and you excite them
1:36:04you say go from
1:36:05the n equal one ground state to the n equal to excited state for instance
1:36:09for the small box
1:36:11what is that look like
1:36:12well
1:36:12you could calculate the energy is based on this expression
1:36:16but you could also just look at the energy levels of both boxes here's any
1:36:19well one for instance
1:36:21for the smaller box
1:36:22and in table two
1:36:24for the smaller box
1:36:25any well one
1:36:27for the larger box here
1:36:30and n equal to for the larger box and years when you notice something interesting
1:36:35if n equal to
1:36:37and you put that into the expression you get a
1:36:40to hear and the two l here
1:36:43that actually energy level lines up with the n equal one case for the smaller
1:36:47box
1:36:48because you'd have a one here and a one here
1:36:53so you get
1:36:54cancellation in the two case
1:36:57for both
1:36:58though larger box
1:37:00and n equal one case for the smaller
1:37:02those energy levels turn out to be the same
1:37:04if you look at the
1:37:06any well for case
1:37:07that lines up in energy with the n equal to case for the smaller but
1:37:12so what you have are
1:37:14a transition between two and four
1:37:17would give you equal energies but a transition between one and for the large box
1:37:22is bigger than
1:37:24one to two
1:37:25for
1:37:25the small
1:37:27so the correct answer here
1:37:28a transition
1:37:30smaller than the transition
1:37:35if you take a particle
1:37:36the behaves like a wave
1:37:38and you trap it you put boundaries on it you trap and in a box
1:37:42it turns out you can only have certain energies
1:37:45it's not like when you take a more able
1:37:47and you put it in a box
1:37:48and you shake the marble around it can have manual kinetic energy it once
1:37:53really fast giving going slowly can be going to continue is
1:37:57amounts of the last these in between
1:37:58that's not true
1:38:00for a particle the bayes like away
1:38:02particles would be like a wave can have only start and energies
1:38:07the energies are quantized
1:38:09remember way
1:38:11plus boundaries gives you quantisation
1:38:14so
1:38:15i have some quantized levels written here and quantisation is the fundamental property that allows
1:38:20absorption and emission alike
1:38:23because
1:38:23if you're going to make the system go from
1:38:26one level to the next
1:38:28you can't absorb just any old wave like
1:38:31you have to absorb the wave like that actually gets that gives the exact amount
1:38:35of energy to go from
1:38:37low state the high energy state
1:38:39all other energy levels
1:38:42and all other energies
1:38:44that the system experiences will be ignored
1:38:46but
1:38:47energies that
1:38:49it's energy map
1:38:50energies that mapped onto this
1:38:53will be absorbed or
1:38:54conversely emitted
1:38:57so here's a wavelength
1:38:59high energy wavelength
1:39:01exposed to this system but there is no
1:39:04spacing energy spacing equal to that energy so that is
1:39:07transmit
1:39:09here's only a wave like that exactly matches
1:39:12this high energy transition that will be absorbed
1:39:16and was absorbed from the spectre you see it's missing
1:39:20in the continuous band that's being exposed to the matter
1:39:24that wavelength is absorbed you'll get a dark area where that we've like there's absorb
1:39:30because
1:39:30the system
1:39:32as an energy weight energy spacing that matches that frequency
1:39:37and of course
1:39:38atomic and molecular transitions work like this
1:39:40you'll have
1:39:41some wavelengths
1:39:43that will pass right through
1:39:44some wavelengths will be absorbed
1:39:47it's like the goldilocks principle of atomic absorption
1:39:50some wavelengths are to be
1:39:52some are too little
1:39:53some
1:39:54are just right
1:39:57so you have another just right that will be absorbed
1:40:00and perhaps of find a third
1:40:03just right
1:40:05energy level that will be absorbed so you get an absorption spectrum that says this
1:40:09would this yellow in this red are removed
1:40:12from the continuous
1:40:14like that's eating this object
1:40:16or you could have the light
1:40:18emit
1:40:19by the system the system could in minute
1:40:22the
1:40:23energy of
1:40:25the highway of like and then you have a single emission it can in the
1:40:28all
1:40:28we've likes of like only sort
1:40:31with length of like are in it
1:40:33and this is why things can have certain colours
1:40:36because the little in emit certain wavelengths of light
1:40:40here at this example
1:40:42a blue wave like
1:40:43a lower energy green wave like and the still lower energy red wavelength
1:40:48absorption and emission
1:40:50of light
1:40:51by objects that are bounded
1:40:54bounded say electrons
1:40:56in a box
1:40:58have certain energy levels
1:40:59therefore
1:41:00certain
1:41:00absorption
1:41:01and emission frequency
1:41:07matter absorbs or emits light based and it's
1:41:10fine electronic structure that is you have electrons behaving like waves there are bounded in
1:41:15the matter
1:41:17so they have only certain energy levels that can exist and all these certain transitions
1:41:22they're allowed to be absorbed
1:41:24automated
1:41:25five four times stimulating that
1:41:28now let's take this
1:41:30let's say a certain piece of matter has this
1:41:33energy level scheme that shown here
1:41:36and it has and emission spectrum
1:41:39which of these to me emission lines
1:41:41arises from the transition of
1:41:43high energy to low energy three to one
1:41:46emission in this system
1:41:47is that a beer c
1:41:50think about that for a second
1:41:51and make a selection
1:42:07let's look at possible explanations for each answer a
1:42:12it is the highest frequency transition so it also has the highest energy and three
1:42:17to one is the largest energy spacing
1:42:19or b
1:42:20means in the middle and transition three to one has an energy level in the
1:42:24middle so there's a similarity between the energy levels spacing is and the spectrum itself
1:42:31or c
1:42:32c is the highest wavelet
1:42:35and therefore highest energy transition and three to one is a large standard space
1:42:39think about those three options
1:42:41and make a selection
1:42:54we're trying to get from the energy levels space things
1:42:58in some matter to the actual emission spectrum
1:43:03that we observe
1:43:04so
1:43:05if you look it impossible energy transitions
1:43:08a transition between level three and two is possible that would be the lowest energy
1:43:13of the possible that the smallest
1:43:15spacing here so that's the smallest energy
1:43:18so on a frequency plot it would be a lowest frequency
1:43:22or highest wavelet
1:43:24if you look at the transition from two to one also possible
1:43:29that some think your energy spacing
1:43:31higher frequency
1:43:32the other transition from three to one is the highest
1:43:37possible for this system
1:43:38so
1:43:39three to one would give you a line at a
1:43:43a here the highest energy transition is the highest frequency is the correct answer
1:43:51let's look at and
1:43:53energy level
1:43:54spectrum
1:43:55so some energy that's in minute or absorbed from a system and try to predict
1:44:01what the electronic structure what this these things are
1:44:05in that actual system of matt
1:44:07so the reverse of what we did last so
1:44:10to which energy level scheme a mere see there's this emission spectrum
1:44:14correspond
1:44:15think about that for a minute
1:44:17and make a selection
1:44:33some possible explanations for each answer a when you flip in energy level diagram ninety
1:44:38degrees that's a good way to arrive at what the spectrum looks like
1:44:43or
1:44:45e
1:44:45too small transitions give the same low energy light and three unique high energy transitions
1:44:51give the higher ones
1:44:53or c
1:44:54there are three large energy spacing and one small and the spectrum has three high
1:44:58and one low energy like
1:45:00think about those three possible explanations and make another selection
1:45:17let's look at the relationship between emission spectra and energy level spacing in the matter
1:45:22so
1:45:22if you have a
1:45:25energy level spacing that looks like a what with the emission spectrum look like
1:45:29we have to look at every possible transition in the system
1:45:33and here you can see three high energy level transitions
1:45:37those would give you high frequency lines
1:45:40and three low you could have this tiny transition this tiny transition and this tiny
1:45:45transition
1:45:45needs to of equal energy so they're degenerate
1:45:48they would fall right on top of each other and give you only one why
1:45:52even though there's two
1:45:54transitions
1:45:55but the two transitions have the same energy
1:45:57so we can't was all of them in terms of energy
1:46:00so you get just two lines one for
1:46:02this transition and one corresponding to both of these transition
1:46:06so that doesn't look like the right answer
1:46:08if we look at
1:46:09b we have
1:46:12one two three
1:46:14for transitions
1:46:16but these and then it'll to
1:46:18are the same energy they would fall right on top of each other
1:46:22so you have a very high energy level a medium and the medium at the
1:46:26same energy and the low
1:46:28in your
1:46:29high band that's a total of three
1:46:32from these for transitions
1:46:34since two are exactly the same
1:46:36and then to tiny
1:46:38low energy transitions but again they are of equal energy
1:46:42so that would give you one line
1:46:44that looks like the spectrum we've seen
1:46:46and if you look at see that of course isn't anything like what we see
1:46:50you have one
1:46:51large energy transition and then
1:46:54two
1:46:55energy transitions in the intermediate range but this one this here and this pair
1:47:01are equal energies will give you only one line so this very gives you one
1:47:05like this very give you one line
1:47:07to intermediate lines and then a small feature
1:47:10so you see i you can
1:47:12analyse
1:47:13at
1:47:14energy level diagram and go to a spectrum
1:47:17in kind of backwards and forwards that's why i s
1:47:20spectrum
1:47:21is valuable it tells you by inference
1:47:24something about the actual matter
1:47:27and remember you can look at tiny little matter in your microscope you have the
1:47:31baby that with this electromagnetic radiation
1:47:34see what it absorbs and the minutes
1:47:36and bile thinking about what it absorbs and the minutes reconstruct
1:47:40what its energy level diagram what it
1:47:42might be
1:47:43what it's electronic structure might be
1:47:46so here
1:47:46the correct answer is b
1:47:54lun is in the demo that preparing to demonstrate laser induced fluorescence
1:47:58fluorescence occurs when high energy photons
1:48:01or absorbed
1:48:02and a lower energy photons are emitted
1:48:05lenny will start with an ultraviolet loser that's higher energy then the visible region
1:48:12as the ultra violet laser passes through this blue solution
1:48:16ultraviolet photons are absorbed and blue photons are emitted
1:48:21here ultraviolet photons are absorbed
1:48:23and photons in the green are emitted
1:48:27here ultraviolet photons are absorbed while four times in the yellow are emitted
1:48:33and ultraviolet
1:48:35photons stimulate the emission of photons in the red region in the final solution
1:48:42now let's try that same experiment
1:48:44with a lower energy loser
1:48:47this is blue in the visible region
1:48:51so
1:48:51in order for fluorescence to occur
1:48:53we have to absorb a high energy photon
1:48:56and emit a lower energy
1:48:58so here blue laser is transmitted through the blue solution
1:49:03but
1:49:04blue photons are absorbed here
1:49:07and photons in the green are needed
1:49:12here
1:49:13blue photons stimulate emission of photons in the yellow
1:49:18in the final solution
1:49:21photons in the blue region stimulate emission of photons in the red region
1:49:29one final laser a green laser
1:49:32so
1:49:32even lower in energy of photons lower than ultraviolet and
1:49:39blue
1:49:39photons
1:49:42so
1:49:44be green laser insufficient to stimulate emission in the blue
1:49:49and the green laser
1:49:51transmitted through the green solution
1:49:57and
1:49:58the green laser
1:50:01stimulating emission of yellow
1:50:03photons
1:50:04from this solution
1:50:07and
1:50:07the green laser
1:50:09stimulating emission of photons in the red region
1:50:12from the final solution
1:50:14so the laser induced a fluorescence
1:50:17demonstrated by losers and a coloured solution