QED - Section 6

There are several possible theories that you could make up to account for the partial reflection of light by glass One of them is that 96% of the surface of the glass is "holes" that let the light through, while the other 4% of the surface is covered by small "spots" of reflective material (see Fig 3). Newton realized that this is not a possible explanation (See Footnote 1). In just a moment we will encounter a strange feature of partial reflection that will drive you crazy it you try to stick to a theory of "holes and spots" -- or to any other reasonable theory!


Another possible theory is that the photons have some kind of internal mechanism -- "wheels" and "gears" inside that are turning in some way—so that when a photon is "aimed" just right, it goes through the glass, and when its not aimed right, it reflects. We can check this theory trying to filter out the photons that are not aimed right by putting a few extra layers of glass between the source and the first layer of glass. After going through the filters, the photons reaching the glass should ALL be aimed right, and none of them should reflect. The trouble with that theory is, it doesn't agree with experiment: even after going through many layers of glass, 4% of the photons reaching a given surface reflect off it.


(from QED, by Richard Feynman, p. 18)
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