Photons (1959)

earlier in this course we found that matter comes in atoms and that currents come in electrons now what about light are there light particles well I'm going to show you an experiment which proves that there are now you remember that we didn't see the effect of electrons till we had very weak currents nor did we see the effect of atoms until we had small amounts of matter and we're going to have to use very weak light in order to see the effect of the light particles now how do we study light we can use a photoelectric cell here is one it consists of a metal plate in an evacuated envelope and when light strikes this plate here electrons are emitted and they're collected by this little electrode in here and the amount of current is proportional to the amount of light striking the tube and if you have very dim light you don't get much current so you need an amplifier now we can use a photo multiplier which is a photoelectric cell with an amplifier built into the same bulb now you can't see how it works here but I can show you on the Blackboard it consists of a number of plates something like this and the light comes in and Strikes the first one and some electrons are emitted here is one for instance it's attracted to this plate by a voltage of perhaps 100 volts applied to it and when it strikes the plate it knocks out four or five electrons and they're attracted to the next plate something like this and each of the electrons that strike this plate in turn knock out four or five electrons and they are attracted to the next plate and so on and eventually you get a pretty heavy current and you can collect it after you've had enough multiplication and with a tube such as we have here with nine stages the multiplication is about a million to check that we can use our sensitive ammeter first to measure the current going into the first stage and then to measure the full output at the collector here that current should be a million times greater than this one let's do it I have here a sensitive am meter which I can use to measure the current going into the first stage of the multiplier let me put the tube out where it gets some light and you see I have a reading and when I keep the light from going into the tube with my hand the reading goes away now that's much too much light to use with a sens I device like a photo mulier so let me turn off some lights now let's see what our reading is it's 5 * 10- 11 amp going into the first stage of the multiplier now we will read the full output of the multiplier so let me connect it up and with the same amount of light I find that I have to go down 1 2 3 4 5 6 poers of 10 to get about the same reading so we conclude that the amplification is about a million as we said now let's put the output of the photo multiplier on the cathode ray osilloscope and now I'll cover it with my hand and then take my hand off suddenly letting the light in there it responds I'll do it again now I'd like to use weaker light and I guess I'd better connect in this little amplifier and you see there are pulses now we have weak light and we see pulses does this mean that light comes in particles well not necessarily because remember how this works light comes in and it makes electrons come out of the first plate and they're Amplified in here and then you see it on the scope and so naturally you see pulses we know that electrons are particles we have to do some other kind of experiment to prove our point now there's another effect of photo multipliers that I want to show you and that is that even when I exclude all the light by covering it with a black cloth you still see some pulses not as many and this background is an effect that is common to every kind of physics experiment fortunately we can get rid of most of it by cooling the photo multiplier and I'm going to do this with dry ice and alcohol let me put the tube in this box put in some dry ice and some alcohol and that will cool it nicely now I've described to you our apparatus what sort of experiment are we going to do well let me explain it to you with an illustration I'm going to assume that you're very fond of milk and that you have to have a quart of milk once a minute and I'm going to supply it to you in a rather strange way behind this wall is a vast amount of milk capable of flowing at the rate of one quart every 10 seconds and once a minute this gate opens for 10 seconds so the milk starts flowing out and at the end of 10 seconds you have your quart in this case the milk is packaged in quart carton distributed along this conveyor belt in an arbitrary way such that one quart goes by any point on the average every 10 seconds this gate opens once a minute for 10 seconds just as this one did now is there any way that you can tell whether the milk is in bulk or packaged just by what happens down here not by looking through the gate or by looking for cartons or anything like that certainly all we have to do is watch when the gate opens the milk starts flowing in this case and it takes a full 10 seconds to get a full quart but in this case when the gate opens a quart might come through right away or two quarts or three quarts or none at all on the average of course you'd get a quart every time the gate is open now you don't know either when the court will come through during the 10-second open time it might come through right at the start or right or at the end here there is is no consistent delay in the arrival of your milk but here you must always wait for 10 seconds after the gate is open before you have your full quart this is the idea that we are going to use to find out whether light is packaged or in bulk here's our light source and here is a shutter which plays the same part in our experiment as the gate did in our illustration it consists of a disc with a hole in it and every time the disc goes around this hole lines up with a hole in this plate and lets the light through it the disc is driven by an electric motor that you see here here's a drawing of the apparatus here's the light source the shutter and the photo multiplier light will reach the photo multiplier only when the holes in the shutter line up this will happen for a brief instant during each revolution of the disc what we are going to do is look for a delay in the emission of photo electrons after the light strikes the photo multiplier now let me line up the whole apparatus and I will use our am meter to tell me when light gets through I'm turning the motor shaft so that the holes line up and light gets through now the meter reading is maximum I'll shut the light off with my hand now that's a pretty strong light and I want to reduce the intensity so that um first let me put on this Shield which will keep stray light out and now I'm going to use these filters which reduce the light intensity by a factor of 100 let me put one in the reading now is the 3 * 10- 4 amp I'll put it in and now I get 4 * 10 - 6 a factor of 100 let me put in another that's about 5 * 10- 8 Amp again a factor of 100 and I'll put in a third well now I have to use so much sensitivity that the meter reading isn't very steady but it's it's about 3 * 10us 10th amp now that's the current coming out of the photo multiplier when there are three filters between it and the light but look this is the output of the photo multiplier V * 10us 10 and amp what is the input that's this current produced by the light and we figured out that this device had an amplification of a million so that that current must be a million times smaller or 3 * 10 - 16 amp now we can express that current in terms of the number of electrons per second it's about 2,000 electrons per second which means on the average one electron every 2,000 of a second one electron per 2,000 of a second now this is just the same sort of statement as one quart of milk every 10 seconds now when the milk came in bulk we always had to wait a full 10 seconds after the gate opened before we had our quart and if light comes in bulk we'll have to wait a 2,000th of a second at this intensity before we get an electron now if the milk was in packages we would sometimes get a quart right away when the gate opened and if the light is in packages we may sometimes get an electron the instant the shutter opens now let's do our experiment I'll turn on this motor which spins the shutter at 60 Revolutions a second and lets the light through for a brief instant 60 times a second now connect the photo multiplier to the cathod ray oscilloscope and so we can see what we're doing I'll take out these three filters here's where the shutter is open now you notice that I've synchronized The Sweep of the cathode ray oscilloscope with the rotation of the disc so that the on period always takes place at the same point how long is that on period Well I can measure it with the cathode ray osilloscope or I can figure it out from the geometry of the disc it's about a 5,000th of a second now let's expand and the sweep and now you see the pulse more clearly let me Mark the start and finish of it from here to here is a 5,000th of a second now I'm going to put in a filter that reduces the light intensity by a factor of 100 I'll need more amplification and you can begin to see the effect of the electrons let me put in the second filter more amplification and now you can distinguish individual pulses notice also that there are some pulses outside the on period this is the background it has been greatly reduced by the cooling of the photo multiplier now I'll put in our third filter the light intensity will then be so low that we'll only get one electron every 2,000 of a second on the average but look this is a 5,000th of a second from here to here a 2,000th of a second from the time the shutter opens is out here well after the shutter is closed again if we had to wait a 2,000th of a second before getting any the electron that is if light comes in bulk then we couldn't get any electrons at all because the shutter is no longer open but we do get them we not only get them in less than a 5,000th of a second but in fact almost when the shut shutter opens let's stop the film and look at a pulse that occurred right after the shutter opened I measured this one and found that it occurred 6 micros after the shutter began to open here's another with a delay of roughly 6 micros seconds we see pulses within times that are small compared to a 2,000 of a second now now what does this mean it means that light does not deliver its energy in a continuous stream but in particles remember our illustration when the milk was in bulk it always took 10 seconds after the gate opened before you got your quart of milk if light is in bulk we would expect 1 2000 of a second delay between the opening of the shutter and getting any electrons experiments like this and many others have convinced us that this is the right interpretation but these particles called photons are not the same kind of particle that Newton talked about for one thing they are intimately connected with waves we're going to show you experiments in which you will recognize the photon and at the same time you will see wave phenomena waves and particles we need both Concepts to describe the behavior of light