Thomas Young's double slit experiment

Hello. My name is Bill Harrington and I'm a graduate student at the Massachusetts Institute of Technology. We're in the Course Six Modern Optics Project Laboratory, and this is Young's Double Slit Experiment, an important experiment from around 1800. We have a laser shining light onto a pair of identical slits. These are rectangular openings in a transparency. After shining through slits, light propagates some distance to a screen where we can observe the resulting pattern. We have a webcam set up so you can see what's happening on the screen. Right now, we have blocked one of the two slits. So light is propagating through a single slit, and this is the pattern that results. Let's see what happens when we open the second slit. The pattern changes significantly. You should be asking yourself why has the pattern changed and why is this experiment important? The answers to those two questions are related. To understand why this experiment is important and what we are observing, you have to go back to the days of Sir Isaac Newton. At that time, people were debating the basic nature of light. One group of people held that light was a particle traveling like a projectile from a source to an observer. Another group of people held that light propagate more like a wave, sort of like the waves you see moving along the surface of the body of water. Based on his own experiments and extensive observations, Sir Isaac Newton decided that light was most like a particle. Newton was quite brilliant in advanced science in a number of fields. So for many people, because Newton says so, was solid evidence that light was a particle, and Newton's corpuscular theory of light was widely accepted. About seven years after Newton's death, another scientist, Thomas Young, was working with light and he came to believe that light behaved like a wave. Young's double slit experiment attempts to address the nature of light by looking at what happens when light passes through a pair of slits. If light behaved like a particle, for the double slit experiment, we would expect to see two overlapping copies of the single slit pattern. This did not match our own observation. On the other hand, if light behaves like a wave, we expect to see the two waves interfere, adding together in some directions and canceling each other in others. If we make some assumptions about how light propagates, we can predict that the intensity pattern on a screen placed far from the slits will be a proportional product of a sinc-squared term due to the width of a single slit, and a cosine-squared term due do the separation of the two slits. This is an excellent match for what we observe in lab, and is good evidence that light behaves like a wave. So the pattern that we observed was due to the interference of light that passed through one slit with light that was passing through the other slit, and it looks as though light is propagating as a wave. Does that mean that we could shine any light through this system and expect to see an interference pattern? For instance, can we shine the light from this flashlight through the double slit and see an interference pattern? It turns out the answer is no and it's a problem of spatial coherence, which is the ability of light from one point on a wave front to interfere with light from another point on the wave front. This laser has a lot of spatial coherence, but a flashlight and most other sources have very little. So how did Thomas Young do his experiment? After all, they didn't have lasers in the 1800s. Well, it turns out it is possible to get these interference patterns to form even when using a source normally considered incoherent, it just takes a little planning and some more work. Thomas Young recognized that not just any light could be used to see interference effects. In this quote from his lecture on the nature of light and colors, he lays out the conditions on the interfering portions of light as deriving from the same origin, arriving at the same point in different paths, and traveling in similar directions. This suggests two possible approaches to the experiment. One approach is to mask your source down to a point. This is probably the technique that Young used as he mentions using a pinhole and a shuttered window in his other publications. It has the advantage of being simple with the disadvantage of wasting much of the source light. Another approach is to simply place a source far away from double slits. This approach allows astronomers to perform stellar interferometry experiments, and will allow you to view interference patterns using street lights at night as a light source. A double pinhole system is quite easy to construct at home, and can produce striking interference patterns using street lights as a light source. To build the system, you'll need a piece of pipe, some cardboard to block most of one end of the pipe, and the pair of pinholes in a piece of opaque material. For my system, I used a piece of stainless steel shim stock, but good results can be had with the material from a soda can. To make the pinholes, the basic technique is to dimple the material with a pen, and then sand away the protruding material on the other side until you have two nice pinholes. More detailed instructions can be found online from the do-it-yourself pinhole camera crowd. Keep in mind that you want your pinholes to be small and close together. Aim for 1/4 to 1/2 millimeter separation and a similar pinhole diameter. Smaller would be better. The assembly of the system is straightforward. Start by mounting the panels to the cardboard so the hole's unobstructed, then mount the cardboard in the tube, and finally, tape everything together. One trick that I found helpful was to use aluminum foil to make the joint between the cardboard and the tube light tight. To use the system, stand somewhere safe, hold the open end of the tube close to one eye, and look at a street light or stoplight at least 10 to 20 meters away. This is a picture I took of a car at a stoplight using the double pinhole system pictured earlier. Notice the strong dark bands in the stoplights and the car headlights. The pattern is even more impressive in person. Warning-- do not use this system to observe laser light or the sun. You may damage your eyesight. OK, so the experiment shows interference and we can even build one of these widgets if we want to see it for ourselves at home. But does light always behave like a wave? What about Einstein and photons? We know that at least in some circumstances, light has to behave like a particle. Well, it turns out something interesting would happen if you did this experiment one photon at a time. We don't have the equipment to do it for you, but we can show you a simulation of what you would see. Right now, you are watching an animation showing what we would expect to see if we could do the double slit experiment with only a small number of photons in a system at a time. The top half of the screen showing the photon impacts at each stage, and the bottom half is showing the accumulated pattern that we would get if we were exposing the photographic plate. For this animation, we are also assuming very narrow and very short slits. So the expected pattern is a cosine-squared variation in the horizontal direction, and uniform in the vertical direction. This animation is based on the results of two historic double slit experiments-- a 1909 experiment which used very low, but not quite single photon levels of light, and a 1973 experiment showing double slit interference in a system launching single electrons. While we wait for the pattern to finish building, I want to thank you for watching and encourage you to investigate further if you have found the material presented here interesting.