The Photoelectric Effect. Light is Particles, not Waves. But wait...... | Doc Physics

that's the vacuum tube right there I'm a big fan of vacuum troops this is not an ordinary vacuum tube though it's a seat Ron see II - a 59 - Rx market value I don't even want to talk to you about how much this thing is worth what I do want to talk about is that it's actually a photoelectric effect tube and there are two principal elements in it the first one is this wire right here and the second one is that big plate right there we'll call it the plate of the photoelectric effect tube and what's gonna happen is I'm gonna shine light on that tube and there's a vacuum in there that's why they call it a vacuum tube some people over in Great Britain call it a valve which is kind of cool too props to them because it does shut on and off the flow of electricity depending on what's going on so it's kind of a clever name like a water valve but we call it a vacuum tube because it's a tube of glass that's evacuated cool so you wouldn't want any air in here screwing things up so that's why it's nice a night but-but-but light can get through here and I've made this little screen so that only light going exactly the right direction gets to go through here and I put this into this box including with a power supply and stuff and basically I'll go from here the real thing to a schematic and we can start talking about what the photoelectric effect is and what Einstein did to make it explained which is pretty darn awesome here's the thing there is a battery or some high voltage power supply or whatnot I don't even care what that voltage is right now and then there's a plate of metal right here which as we saw in the tube was a curved piece of metal and we're going to call it no sorry that's the collector is that wire that's very thin out here this is the collector and that's that's on the front of this tube as you can see right now and then we've got the emitter over here and our emitter was a curved piece of metal a design I mean I'm gonna kind of curve this one so that you see that the incoming light probably we're gonna have light coming from this direction and it's all wavy right we got waves of light coming in this direction and I'm gonna call them photons because well this is really the essence of our problem here are they photons or are they waves and who knows what's going on but this is our emitter and the emitter is connected to an ammeter right here the ammeter will measure if there's any current going through I guess if there's current going through here then they'll be current going through here and here and here so the ammeter is perfectly situated to tell us if there's any current in this photoelectric tube and then we have to say that all of this stuff will be encased in glass I need the collector and the emitter to be encased in glass all right the idea is this when light hits the emitter plate light comes in with some energy and just like you when you bail out your uncle again from jail as soon as the jail is paid this makes sense I don't know kinda I don't know a lot about criminal justice but as soon as the jail gets their money then your uncle can go free maybe he'll have to stand trial and stuff too but if there's sufficient energy in the light to free that what what do you think's gonna get knocked off you know the structure of an atom right the structure of atoms like you know some neutrons and some protons and then way out here I'm talking like way out here and I don't really mean way out here like this I mean like several rooms away at the scale way out there there's an electron doing something guess who's gonna get freed by the photons coming in by the light coming in its the electron of course don't be silly so I'm saying that if there's sufficient energy then electrons will be freed and the cool thing about electrons is if they escape right here excuse me if they escape the emitter then they will be drawn to the collector because the collector is a positive place to go to and the emitter is a negative place to be so they're going to be very excited about leaving and in this tube that looks like they leave this plate right here and they go to that particular wire that then collects them and dumps them over there into the power supply and ultimately through the ammeter so we see some current so we're gonna be able to change the conditions of the power supply and measure the current that goes through whether it is zero or big or I guess it couldn't really be negative but we're gonna get constant so here's what I'm saying I'm saying that these electrons will be freed if sufficient energy exists to free them they are trapped in their atoms on the emitter and they can be knocked out if photons have enough energy there are two predictions that classical physics makes those predictions are as follows one classical classical physics says that regardless of the color of light that you shine on the emitter there will be electrons ejected as long as the intensity is enough so okay let me see if I can summarize that regardless of frequency electrons emitted if amplitude cuz it's a wave right is big enough that's my first classical argument and that has to be the case because we've spent hundreds of years developing the idea that light is a wave back to heavens principle and all of this stuff comes to the fact that light is in fact a wave and not particles or bullets of light or something regardless of the frequency electrons will be emitted if the amplitude is big enough we have a freaking wave people and that wave is doing this if it's not a lot of intensity and it's doing this if it is a lot of intensity so this blue wave ought to be able to knock out electrons much more effectively than this blue wave right here secondly watch this classical physics also predicts that maximum amount of kinetic energy should increase if the amplitude is increased so that means if the wave is bigger we're going to emit even faster electrons that's also very reasonable right so any given electron seems like it could have an arbitrarily large amount of kinetic energy if we're putting in a really high amplitude wave let's go crazy we're putting in a really high amplitude wave we could knock out your uncle I mean good luck knocking out your uncle we could knock out an electron with an enormous Li high kinetic energy because that wave is hitting the electrons and it's making them zoom out really fast so let me see if I can summarize this in a handy way k max gets bigger as hmm K max gets bigger as what Oh as the amplitude gets bigger all right I'm going to take a break I suggest that you pause it and you guess what's going to be wrong with these two predictions you kind to get the feeling that they're all completely nuts but I want you to try to figure out what's actually happening based on a quantum understanding of light if these light waves are in fact particles what would that mean for the frequency and the amplitudes relationship and what would it mean for the maximum kinetic energy I'll tell you so you don't have to wonder too long but go ahead pause it again I just had a sausage biscuit from McDonald's and I'm ready to go now these classical predictions are nuts they're completely wrong regardless of frequency an electron is emitted that the amplitude is big enough if we've got particles what the heck does amplitude even mean increasing the amount of light that hits the emitter means sending more particles in and I think Einstein just finished saying that E is H times F so in fact the energy of every single particle is simply proportional to the frequency of the light that you're sending in so if you want more light you don't increase amplitude you increase the number of photons and you can send in photons individually or you can send in billions of them at a time right now you're getting hit by billions of billions of photons from your computer screen or what you're watching this on your phone aren't you cool okay and look at this K max gets bigger as the amplitude gets bigger that's insane because these photons will be interacting with one electron and they will be sending their energy to one electron and if you get more of them that's what it means to get amplitude bigger more photons will simply interact with more electrons and you will get a greater current but you won't have them going faster asterisk how tell how the heck do we tell how fast they are going and I'd like to go into that by drawing this a little bit differently regard this battery we're going to the next page I'm gonna draw a completely similar photoelectric effect setup and I'll have the battery over here we'll deal with that later here's the collector here's the emitter we've got an ammeter here and the battery is facing the wrong way the battery says no don't bother coming over electrons I'm busy so here is an electron like here these packets these are these waves that are going this direction but actually they're like wave packets they are little bits well they're photons let's be honest with each other I don't know what the heck that means but they are particle wave things they're little blobs of energy and their energy is H times F turns out Einsteins right so what they're doing is they're hitting this emitter and then an electron is freed and chances are that electron will go any which way that's fine it just got freed by the aware the photo in the photon it becomes what we call a photo electron just because it's a regular Ektron that got freed by a photon and that if this vote it's electron is going the wrong way it'll just go pump remember check this out this is going to be negative now and this side is going to be positive sorry about my throat Wow maybe I should eat more healthfully before making these lectures then let's say this other photon comes in and hits this electron and this electron just happens to be aimed exactly the right direction it's like BAM and this voltage here between there and there there's a voltage yeah it's the voltage between this plate and that plate we could call that a stopping potential in fact that is what it's called stopping potential is the voltage between those two plates and it says to this electron I don't want you to come over here if you do successfully make it over there then we will have a current but there's no light coming on this part right here so there no way the electrons can come off of here and like do the effect backwards so the only way we're going to get a current is if electrons do successfully a get freed from the emitter and B happen to be pointing exactly the right direction and C be going fast enough so that their kinetic energy will transform into potential energy and they will actually plump on - is that a word yeah on to this collector right here and that will be a measurable current you can imagine there's a stopping potential where I can stop everything there's also a stopping potential where I get just a few of them through and that's what I'm particularly interested in the stopping potential where they only just barely make it and that's what I'm talking about with K max the maximum kinetic energy then must be the charge times the stopping potential Q times V because that must be the change in potential energy of these suckers to go from here to here they're gaining potential energy to go to this negative spot where they don't want to be but if some of them happen to be pointing exactly the right direction and given exactly the right amount of energy then they will make it over there that's how we can measure the maximum kinetic energy of these suckers so here's my plan I'm gonna refute both the classical predictions and tell you what actually happens and then we'll see how quantum can explain it this is the real result of this experiment one first of all the light has to be the right color to eject electrons you do not get any electrons out even with no stopping potential you don't get any electrons out unless the light is the right color in fact there is a cut-off frequency and you know color is correlated with frequency of course so that means there must be a cut-off energy that energy is a very interesting energy so let me first say cutoff frequency cutoff frequency and I'll call that F naught that is the maximum the minute the Makah minute not met the minimum frequency you have to have to free these suckers what I mean is it's the minimum energy that you need to have we could put little knots on here the cutoff frequency and the cutoff energy cutoff frequency or higher needed to get any current whoa that is incredibly profound they're saying regardless of how bright the light is if I come in there with these different colors of photons and I'm sending in a red photon for instance it may not admit any electrons at all and then I switch to pink law I don't even know what that's doing in there it switched I switched to orange it doesn't emit any and suddenly at yellow it emits some and the interesting thing is at yellow it emits exactly the same number as at blue for the same brightness of light oh that's really interesting and purple also lets the same number of electrons out so there's like you've got to be at least this high energy in order to free something that's like having just enough money to get your uncle out of jail this is related to the fact that these electrons are bound in here and if you don't hit them hard enough they cannot get free there's a wonderful video that says that there's a pit and the electron is a ball in that pit and if you hit that ball then it can make it out if you don't hit it hard enough it'll just go whoop up the edge a little bit and roll back down the electron will remain trapped but if you hit that sucker hard enough it can go down and it will be free and it's maximum kinetic energy is given ba we'll see a cool equation for the maximum kinetic energy but I want you to understand that color of light some colors emit electrons and others don't and it also depends on what material we've made this emitter out of whoa that's kind of funky all right so let's check for number two but just to re-emphasize the number one brightness doesn't matter it is only frequency whoa that flies in the face of all classical physics because brightness is amplitude see the amplitude right there brightness is amplitude in classical physics and if we talk about particles of light we have quantized them we're doing quantum physics then we say that brightness is quantity it's a whole bunch of photons rather than a whole big intensity of light meaning a big amplitude of the wave the second observable fact in the photoelectric effect and we can literally do this in AP Physics lab ain't no thing the second observable fact is that if the frequency is greater than the cutoff frequency increasing the intensity like what I mean is we have enough energy to free these suckers if we've got at least this much energy so I'm gonna say if energy of the light is greater than the cutoff energy of the light or oh sorry the parentheses should start there or F is greater than F not increasing intensity increases number of electrons not the maximum kinetic energy of the electrons this also doesn't make any sense to a classical physicist because she says let's see if we increase the intensity then we'll be increasing the amplitude so we'll be more likely to free electrons and they'll be coming out faster as well but the quantum physicist says no you've got that wrong you need to understand that if I'm increasing the intensity I'm simply sending more photons in and so I will get more electrons out but they will all be going the same maximum speed I'm saying K max because there could be some other things going on they might be going your directions and there might be some weird absorption or something but there's a final awesome thing that comes from this photoelectric effect so both of these say quantum is real and shove it essentially but let's get a little more space over here and I want to say that you can make a graph now I love graphs I'm going to make a graph of well it's going to be frequency on the x-axis and it's going to be the maximum kinetic energy which remember we're measuring by the stopping potential these super stubborn electrons are going to go the wrong way because they're kicked out of their emitter with so much kinetic energy that they do actually manage to go uphill energetically speaking and cause a current even when there's a stopping potential on so we can measure the maximum kinetic energy and the cool thing is the brighter the frequency these electrons do come out with a higher did I say that right what I mean is the higher the frequency they come out with more kinetic energy so you make a little graph right here and you're thinking well if I use a sodium check this out if I use a sodium piece of metal in my - and that would be okay sodium is not very good if you're in air but I mean you could keep it clean in a vacuum right if you use a sodium piece of metal you might get this pattern right here you get some data and you find whoa if I increase the frequency then I'm getting more and more energetic electrons spit out of course if I get to this point right here this I'm going to define as the well the threshold frequency because at that point I'm getting canet maximum kinetic energy of zero I'm not getting any electrons out right here so then you'd find that to be the case and if I take a different metal let's say that I take a metal like gold or something and I make it out of gold I get a very very similar pattern right here but it's got a different what are you gonna say it's got a different x-intercept which is f naught for gold and I mean we could look at this equation right here this is a very interesting equation but with this threshold frequency we must mean something very different I guess what I'm saying let me define threshold frequency here threshold frequency is defined as well they call this energy the threshold energy they call it the work function that's the energy that it takes to get the electron from here to here and they use the letter v for it I don't really know why but I'm just doing algebra on this guy right here so I'm saying F is Phi divided by H so this defines what's called the work function and you should think of the work function as the amount of energy required to bail your uncle out of jail if the work function of energy is given to your uncle then your uncle will be free now if you give your uncle more money than the amount of money required to get him out of jail then he will leave jail with some money there's no problem with that giving your uncle a little bit extra spending money maybe he needs to get some new pants I don't know orange looks good on him but not that good and you know he doesn't want to wear what he came in with so work function is what I'm trying to say is work function is the minimum amount of energy your uncle needs if you give him more energy that is you give him a higher frequency Photon then he will leave with more energy than well he will leave with some energy help some leftover energy and I guess this something to do with this energy right here we could get an equation that acts like that and I'm gonna say that the maximum kinetic energy watch this K max is well it's the energy of the photon minus the work function our book actually uses a W naught and I have never seen that anywhere else so I'm not going to use it I'm gonna use the standard 5 the maximum kinetic energy of an emitted electron is the energy of the photon that came in minus the work function of the metal and the work function of the metal isn't on this graph but if I'm thinking about this graph in terms of y equals MX plus B I want you to tell me where I can find the work function I need you to look at this as if you just made this graph and that graph and remember these graphs are actually stopping right here they just there is nothing going on below that but I want you to find Phi the work function good luck please remember that this is a big part of understanding modern physics and why quantum mechanics was necessary do some problems please because you'll find the units of maximum kinetic energy are generally given in electron volts if the units are electron volts then I suppose look at that isn't that cool one electron has an electron volt of one if it goes through one volt potential so we can talk about its energy in terms of one electron volt if it manages to escape a one volt stopping potential that's a really handy unit for the maximum kinetic energy you could put it in electron volts right there and that's probably how problems will be written for you you should be practicing those stop watching videos do some problems good bye sorry one final thing on that graph we made graphs of kinetic energy maximum versus frequency and they went like that what's the slope here get it

Channel: Doc Schuster

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Length: 24min 8sec (1448 seconds)

Published: Fri Apr 05 2013