PHYS102: Introduction to Electromagnetism

What makes the double slit experiment better than diffraction by a single slit, if you want to prove the wave nature of light?

It may still be possible to explain why you can see a light beam spreading out behind a single slit, by photon particles ricocheting off the edges of the slit. But when the spreading light waves coming from two slits overlap, they form the characteristic pattern of constructive and destructive interference that only waves can produce.

Moreover, because this interference is created by two waves that are, at any given point, almost parallel, the corresponding interference patterns are spread out spatially. This is important because it magnifies the interference pattern from a microscopic to a macroscopic scale that can be seen by the naked eye.

You can see an analogy of the double-slit interference pattern when you hold two combs on top of each other against the light. The teeth (or tines) of the comb are like the wavelength-spaced ripples of a wave. When the combs are perfectly aligned, you can see through the spaces between both sets of teeth, but when you rotate one comb ever so slightly, you will see the light being blocked in regular patterns that form the analogue of destructive interference between two waves. The more you rotate one comb, the closer the spacing of the dark pattern becomes.

Rotate the combs back to a nearly parallel angle and you see the spacing between the dark lines grow. The latter is what happens in the double-slit experiment if the separation of the slits is decreased, because it brings the diffracted waves from each slit closer and closer into alignment.

Watch this video for a step-by-step construction of the interference pattern.