The Interference of photons

we have studied waves in considerable detail and we have found that they seem to describe the behavior of light but in the last film I showed you an experiment in which light seem to behave like a particle we now have two ideas about light waves and particles which is right well in some sense both ideas are right and to show you this I will do an experiment in which the wave properties of light and the particle properties are seen at the same time here's our apparatus it consists of a light source with a slit in front of it the light comes along here and goes through a hole in this baffle which is designed to keep out stray light and then the light strikes this double slit here which can be moved back and forth by means of this electric motor and this mechanism and I have this pointer here to keep track of its position when the pointer moves in this direction the SL slit is coming towards me and when I reverse the motor the pointer reverses and the Slit moves away let me stop it here then the light goes along and if I turn on the light switch I can look down in here we can see the interference pattern we're all familiar with bright bands where the waves reinforce alternating with dark bands where they cancel this is a typical wave phenomenon from the geometry of this arrangement we can calculate the wavelength you've done it yourself in the lab if I move this double slit the interference pattern moves now if I put this slip in here like this then when the interference pattern moves back and forth more or less light will get through that slit into our photo multiplier which I can put here this is the same photo multiplier that we had in the last film here's what happens here's our light source with the slit in front of it and here's the box and here's the photo multiplier with its slit and now I can draw the movable double slit on this plate of glass somewhere in the middle like this and then light comes along like this and goes through through the double slit and forms the interference pattern down here so that there are dark bands alternating with light bands something like this now when I drive the double slit back and forth I sweep the interference pattern across the photo multiplier slit now for instance light is getting through that slit into the multiplier now there's no light now there's Light No Light so that we expect that the output of the photo multiplier would consist of Maxima and Minima when we drive the double slit back and forth of course we could have moved the photo multiplier or the light source but it was more convenient to move the double slit back and forth now let's see it happen I'll connect the output of the photo multiplier to this meter and then when I run the double slip back and forth the reading goes up here's a maximum now it's going back down to almost to zero and there's the minimum and let me leave it on a maximum now if we're to see the effect of photons we have to use dim light as before and the first thing I must do is to cover this apparatus to keep out stray light I'll diin the light source and I must use more and more sensitive ranges of this meter and now the current coming out of the photo multiplier is about 10us 9 amp let's remember that number we can put the output of the photo multiplier on the cathode ray oscilloscope and you see we have the same kind of pulses that we had in the last film besides looking at them we can listen to them I can use this amplifier and this loudspeaker if I turn the light off all you hear is an occasional background tick [Applause] now let me move the double slip back and forth and you can keep track of its position by looking at that pointer now you remember that we left the slit at a maximum of the interference pattern in fact it was the central maximum of the interference pattern that you saw a moment ago now I'll start the motor and you notice there are fewer pulses the sound is coming down here's a minimum now it's coming up again this is the first Maximum a minimum and the second maximum is lost in the background now I'll reverse the motor we'll go back through the pattern is the minimum and here we're coming into the first [Applause] Maximum another minimum and here's our Central maximum we're going on path the pointer is in the middle here's the first Maximum on the other side minimum and again the second maximum is hardly distinguishable we have an interference pattern we could have plotted it just by listening to the sound now let me bring it back to the central maximum and look at those pulses listen to them they are caused by pH phons we saw that in the last film when we found no delay between the emission of electrons and the turning on of the light photons particles are forming an interference pattern a characteristic wave phenomena now look at this you remember that the output of the photo multiplier was about 10- 9 amp and and that it had an amplification of about a million so that the current coming in here must have been about 10- 15 amps that's 10,000 electrons per second not every Photon that strikes the photo multiplier produces an electron some get lost on the average only one out of every thousand photons succeeds in producing an electron that is 10 3 photons per electron so that if we have a current of 10,000 electrons per second there must have been 10 7th photons per second entering the photo multiplier that means that on the average there is one Photon coming in every 10us 7 Seconds these photons travel at the speed of light because they are light so that by the time a photon has reached the double slit the one that preceded it has been absorbed in the photo multiplier in fact if the photo multiplier weren't there that Photon would be 100 ft away as you can easily figure from the speed of light and the time between photons this means that there is rarely more than one photon in here at a time let me go over that a photon comes along here all by itself goes through the double foot continues along here still by itself and ends up in a bright part of the interference pattern never at a minimum now this may seem strange to you but it's the way light behaves what can we say about it well here are some waves and you can see them and here's a particle it's tangible but can you see the waviness of light can you handle a single Photon no we observe the behavior of light and compare it with models this model for waves and this model for photons but neither model is capable of describing the behavior of light accurately by itself baseballs don't show wave properties water waves don't act like particles nothing that we've discussed acts like particles and waves at the same time but light can and does all the experiments that have been performed show it how do we interpret these results well where there's a maximum of intensity in the interference pattern on a wave theory we are most likely to find photons where there's a minimum we find none with this description of light we are able to predict Its Behavior and for the moment that means we understand it