Wave-Particle Duality Explained with Double Slit Experiments - Christmas Lectures with Neil Johnson

now story starts with a duck pond and with this man Thomas Young was about 200 years ago he was sitting by a duck pond watching some ducks I have here a pair of ducks we'll have a look at what Thomas Young saw just wind up my ducks what do you saw as the Ducks swam around was a pattern appear in the water which you can see down here now the pattern was caused by ripples on the water leaving the ducks and interfering with each other it could sit in a more controlled way here we have a motor there's actually going to cause ripples and it's going to pass through two slits and these slits are going to produce ripples which go out into the water and actually interfere very much like with the two ducks and you could see a pattern emerging you can see here along this line two ripples are adding up so you're getting bright and dark lines but here doesn't look like anything's happening that's because the waves the ripples have actually cancel each other out now this made him pretty happy first look as he like ducks but second of all because it reminded him an experiment he'd done a couple of years earlier with light I'm gonna show you now what he saw with his light experiment here we have something very similar to the pond and the two ducks actually very similar to that tank with water and the two slits because here we have something that's produces light and the light passes through two slits actually very close together because light has a very short wavelength a very small wave so you can't actually see these two slits but it's the same idea and then the light passes on and hits this screen you could actually see a pattern it's quite similar to what we saw on the on the tank with the water and there are regions of bright and dark and bright and dark and bright and dark so this said him thinking well if I saw the same pattern as I saw on water I know that waves travel on water that must mean that light is a wave after all particles wouldn't do this have a look at what particles will do for this I need a volunteer with a lot of puffs could you come down your name is Jay could you stand here now what you're going to do is going to blow through this tube and here is sand nao-san cuz this the grains particles lots of them and they all packed in here and as Jay blows he's actually gonna blow sand through this box up against these two slits and the sand can pass through these two slits very much like two slits and we've had with light on the back here we've got a screen we're just going to collect the particles of sand as they go through the slits so huff and a puff and blow through the tube Jay keep going great you can do it a little bit yes that's it that's great thanks very much go back to see I'm just gonna show you what's on the screen and this is it it's not really a surprise there are two stripes very much like the two slits of course sand consists of grains like little particles and they either go through one slit or they go through the other slit and that's what you're seeing these particles have gone through one slit these have gone through the other slit so this is what sand does and this is what particles do they produce this pattern and it's unlike the pattern that you get with waves now we're going to try something a little different we saw the pattern that light produced nothing like the sand but here we've got essentially the same piece of equipment as before but we're going to turn the brightness of the light down here we have the source of light over here and it's passing through two slits we've had to cover this up because we don't any light to get in the light passes through the two slits goes over to the other side instead of a screen over here we've actually got a camera because this detects really low levels of light I'm gonna see what we see actually up there on the screen you can see what the cameras picking up the first surprise it's just picking up dots blips this is light traveling through this but it's arriving as blips like the sand there's particles so that's amazing but have a look at this if we collect up all these blips collected over a time period and have a look at what the pattern is we see something quite remarkable the pattern you see is the pattern that waves make but the light arrives in blips in packets in small particles which physicists call photons so this is incredible because the lights traveling making a pattern as though it were going through two slits because remember to make that pattern that wave pattern they've got to be two sources two slits the lights got to go through both slits and yet it's arriving as a blip as a particle like the sand this is the essence of the weirdness of the spookiness in the quantum world you

Channel: The Royal Institution

Views: 89,646

Rating: 4.8074074 out of 5

Keywords: Ri, Royal Institution, double slit experiment, wave-particle duality, waves, particles, quantum physics, light, physics, neil johnson, christmas lectures, science experiment

Id: O81Cilon10M

Length: 7min 3sec (423 seconds)

Published: Fri Feb 01 2019