The Bakerian Lecture: Experiments and Calculations Relative to Physical Optics

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   Throw balls through a board with two slits and the wall shows two stripes; send waves and it shows many bands, bright and dark, because waves add up and cancel out. Light makes bands, so it behaves like a wave.

   The Bakerian Lecture: Experiments and Calculations Relative to Physical Optics · 1804

   Balls make two stripes, waves make many bands, and two lights added together can make darkness.

   Before any mystery, you need to know what you would expect to see. Picture a board with two narrow slits side by side and a wall behind it. Throw tennis balls through the slits and each ball goes through one slit or the other. Two piles build up on the wall, one behind each slit. Nothing surprising. Now put the board in a tank of water and make waves. Each slit becomes a new source of waves, and the two sets of waves meet. Where a crest meets a crest, the wave grows. Where a crest meets a trough, they cancel and the water stays still. The wall shows a pattern of many bands: strong, quiet, strong, quiet. This is called interference, and it is the signature of any wave. Around 1801–1803 the English physician and physicist Thomas Young did this with light. He split a beam of sunlight in two and saw bright and dark bands on a screen. His conclusion was that light is a wave. Notice one thing that seems strange: light from both slits reaches the dark bands, and yet they are dark. Two lights added together can make darkness. Balls never do that. Remember this difference, because the rest of the chapter rests on it.

   “I think it will not be denied by the most prejudiced, that the assertion is proved by the experiments I am about to relate, which may be repeated with great ease, whenever the sun shines, and without any other apparatus than is at hand to every one.”

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