## The strange theory of the photon

We have seen that not only can light behave as a particle, but that this particle of light, the photon, has properties like no other particle. It seems to go from place to place by more than one route at the same time; it seems to 'know' if another route is closed or open, even when it apparently has not travelled by that route. Its rules for reflection follow a strange kind of arithmetic.

We can extend Young's experiment with two slits to three or four or more slits, in fact to any number of slits, until we have a diffraction grating with tens of thousands of slits. (Chapter 8). A single photon now has all these possible routes available

single photon diffraction grating screen

Figure 14.10 A single photon faced by a diffraction grating. There are many routes available to go from A to B.

through the diffraction grating, and somehow they all contribute to the interference pattern on the screen!

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