this is a demonstration I did for my second year quantum mechanics class this morning the moment were talking about wave particle duality and they're all familiar with the electron being a particle it's what you learn in high school and even in first year and then in second you suddenly get confronted with this idea that the electron can behave like a wave and so I wanted to give them some real experimental proof that this actually happens we brainstormed out a few different ways of doing this one way is to do Young's double slit experiment with electrons and that turns out to be rather hard to do you can do things like optics for example and again that's hard to do but it allows you to make things like electron microscopes for example one of the easy ways to prove that electron behaves like a wave is to do diffraction some of you will be familiar with Bragg's Bragg diffraction in materials and so what you have is you have a crystal with a set of planes and if you shine an x-ray in at some angle that x-ray will reflect off the surface and reflect off the next plane and so forth and those reflections can interfere with one another and you'll get a set of interference maxima and minima they'll allow you if you know the wavelength to be able to determine the spacing between the crystal planes now if an electron is away you should be able to do exactly the same thing and the apparatus that I have here effectively does exactly that okay the configuration is slightly differently but it is slightly different but the principle is exactly the same so let's have a look at this piece of equipment very similar to picture tube in your television at home assuming that you haven't spent your kevin rudd bonus on buying a brand new plasma television in which case it's no longer true if you have an old-school television and you break it open on the inside you will find a glass vacuum tube much like this one there will be a screen at the front and you'll have an electron gun at the back now the electron guns fairly simple piece of apparatus what you have is you have a filament so it's just a small piece of wire just like in a light bulb you pass a current through it and it heats up and when it gets hot there's enough energy in there for electrons to jump out of that filament okay so there's electrons jump out at filament and then they'll quite happily jump back in again but if you have positively charged plate somewhere over here when the electron jumps off it goes hey there's a positively charged plate I'm going to head over that way so it starts accelerating off in that direction and if you arrange for your plate to have a slit in the middle of it you can have your electron will come along attracted towards that plate and it will go straight through the middle and come out the other side okay and what this allows you to do is take electrons and speed them up to some velocity and fire them and things literally like a gun okay so the electrons Funds starts at that point by the time it goes through this positively charged plate it's traveling at a hundred thousand meters per second or so and then what we're gonna do is we're gonna slam it into a target okay and that target is going to be just a mesh coated with graphite powder and graphite has a really interesting structure it consists of a set of sheets each of those sheets has a set of carbon atoms in them and they bonded together in a hexagonal arrangement much like chicken wire for example in each of those sheets stacks on top of each other to make a piece of graphite okay graphite is really nice in that sense that the planes can slip on top of each other and that's why graph graphite gets used as a lubricant quite a lot so if you're for example lubricating a lock when it gets a bit sort of gummy or you might do is take a pencil and shave the graphite off the end and take a paintbrush and sort of paint that graphite powder onto your lock the planes slip against each other and make a good lock lubricant and your lock moves really easily again okay now the nice thing about these stacked sheets as they give a set of planes for the electron to diffract off okay and so your electron will start at this filament on the back it will get accelerated by a voltage across a set of plates in the middle and get slammed into this target which is right at the front here and then the electrons will go through that Targo and come out the other side and they will travel through this space here now this space here is in a vacuum we've taken all the air and water molecules out so the electron doesn't to them and get distracted from what we wanted to do which is to end up on this screen at the front the screen at the front is much like the one on your television at home it's a material known as a phosphor and in a phosphor when electron strikes it gives out a little bit of light okay and so we will have electrons crashing into this screen at the front and they will start giving out light out front now in your television at home what you have is you have three different kinds of phosphor and they go in different colors when they get hit by electrons so one will be green one will be red won't it be blue what you do is you take little spots of this phosphor and you arrange them in little triangles red green blue and you make a triangle go all the way across your screen so it's red green blue red green blue red green blue and you take the target out you don't want that because it's gonna mess up your image but you still have this electron gun at the back and what you do is you add an extra set of plates in here that then allow you to scan your beam backwards and forwards so you go left right left right left right run down the screen go to the top do it all again and you turn your electron beam on and off and you can slowly build up a picture by hitting the red green and blue pixels right so imagine you want to make a white pixel you'll hit red green and blue if you want to make blue you will just hit blue and you'll have it off when you hit red and green you can make any color you want by adding those colors together it's a television set right this is fairly similar similar okay so coming back to the idea of the experiment we want to try and determine whether the electron is gonna behave like a particle or it's gonna behave like a wave so let's look at what behaviors should have if a base like a particle it should come through here should smash into this graphite target come out the other side and the particle should just scatter all over the place and so the image that we should see on this screen at the front will just be this blurry green mess where all the electrons are hitting at random on the front of the screen if the electron behaves like a wave it should get diffracted by the planes in the graphite again so what should happen is that for certain directions the electron wave will interfere with itself and will interfere constructively and so we'll get a bright spot on the screen and in other places the two waves should interfere destructively and we should get a dark patch all right it's all just about whether you of that path difference is equal to an integer number of wavelengths if it's equal to an integer number of wavelengths you get constructive interference if it's equal to if it's not equal to an integer number of wavelengths you get destructive interference so if you look at the front of this screen we should get a bright green spot right in the middle which corresponds to electrons have just punch right through and keep going in smashing to the screen and then moving away from that we should see a dark band where you get destructive interference and then a bright band where you get constructive interference and dark band and a bright band and these things just form rings around this central spot okay so this experiment allows us to tell is the electron a particle or is a wave by whether we see fussing image on the screen or whether we see rings so let's fire it up and have a look at what happens so the first thing we have to do is we have to supply power to the filament so we can turn that on it's going to be rather difficult to see on camera even if I do a close-up but it's just here in the back and it'll be glowing bright red hot if you go home and you've got an old TV you can look through the vent on the back of it and you can sort of see inside the box you might notice that there's a glowing red light inside that'll be the filament in your electron gun and then the next step of this process is to put a voltage on the set of plates that are in front of the filaments so that we can take this electron that comes out provide a positive volt charge out here and attract it out okay and by setting that voltage we can control the speed that the electron travels with which allows us to control its momentum and if you know the de Broglie relation it relates the momentum it says that momentum is equal to Planck's constant via by the wavelength so the higher the momentum we have which is the higher velocity we have the smaller the wavelength gets right and so we should see the fraction in this experiment when the wavelength gets small enough that it's comparable to the separation between the planes in the graphite so what we need to do is turn on the voltage here and we need to turn it up to a fairly decent voltage a couple of thousand volts in order to get the electron to be traveling fast enough that its wavelength is comparable to the plane spacing in graphite but as you do it at first the first thing you'll see is there's a little bright green spot turning up on the screen it's just there and as we turn the voltage up higher and higher these rings will emerge it's gonna be rather difficult to see from this angle because unfortunately I don't have blinds in my office but we'll go to a close-up and we'll have a look at those rings