Young's Double Slit Experiment

The most famous experiment demonstrating the wave nature of light is the Double Slit Experiment. Its genius lies in its simplicity.

Thomas Young set up this simple experiment back in 1801.

Young shone a lamp emitting only one wavelength (color) through a screen with two narrow slits cut into it. Behind the first screen Young placed a second, viewing screen to see how light came out after passing through the two slits. This second screen kind of "catches" the light that passed through the two slits. He looked at the result and was amazed. What sort of pattern does light make on the second screen after passing through the slits?

If light were (only) a particle, we should just see two bright lines that match the first screen's slits on the viewing screen. We can imagine it as taking a spray paint can and spraying paint through the two slits. Most of the paint would stick to the outside of the slit screen, and some of the paint would make it through the slits and hit the second screen.

Well, that's not what Young saw.

What did he see instead? He saw alternating lines of light: light, dark, light, dark…and so on. But particles don't interact that way, particles behave like the paint would, making only two lines. Particles do not spread out from the slits the way that Young observed, either, but waves…waves do that.

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This dark, light-dark-light pattern can't be explained by the particle nature of light, which points towards the wave nature of light.

The reason we see the light-dark-light pattern is because of areas of constructive and destructive interferences. Also, waves, unlike particles, diffract.

When the light waves squeeze themselves through the slits on the screen, they diffract around the edges of that slit. The slits essentially create two circular wave fronts, free to interfere with each other and create those bands of light and dark.

Check out this movie of a Double Slit Experiment. Wherever waves interact, or interfere, constructively, the wave becomes dark orange in this movie. Wherever the waves interfere destructively, and cancel each other out, you get yellow. When we look at the wall on the very right of the movie, we see spots of dark orange hitting the wall: these correspond to spots of light. We also see spots of yellow on the wall and these are the dark lines resulting from waves that cancel each other out.

We remember from the wave chapter that when two waves meet they interact with each other. As long as they're the same sort of wave they just add themselves together. If two waves meet peak to peak, we get a wave with a peak twice as high (twice the amplitude). If they meet peak to trough, we get a smaller wave, or no wave at all if the two waves have the same amplitude.

The wave chapter also told us that only light waves of the same wavelength can interact with each other. Red light, for example, happily interacts with red light, but it won't mix with blue light.

This like-interacts-with-like behavior is what makes Young's decision to use monochromatic light (a.k.a light of just one wavelength) so brilliant. If he had used regular white light, which contains all the colors of the rainbow, light would still have behaved like a wave, but the different wavelengths would have different places of constructive and destructive interference, muddling up the individual interference because of the number of wavelengths involved.

If this were any other subject, the story of Young's double slit experiment would end here, but nothing is ever that simple in physics.

Along came a young lad by the name of Albert Einstein, you might have heard of him. He showed that light is actually a particle with wave-like properties. That makes light both a wave and a particle. If this sounds like nonsense at first, then rest assured that almost everyone else agrees. Wave-particle duality is one of those facts of life that we just have to accept, like that 1 + 1 = 2 or that our rooms won't clean themselves.