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One experiment one could imagine doing which would provide
conclusive proof of a particle nature of objects is to scatter
two objects off of each other, as in the collision of two billiard
balls. This experiment with light and small atoms has been done,
and is now called Compton scattering. The basic setup is
The results of this experiment would be completely at odds with
predictions made if the light was viewed as a wave - for example,
one finds experimentally that the frequency of the scattered wave
is changed, which does not come out of a wave picture of light.
However, when the light is viewed as a photon with energy
proportional to the associated light wave, excellent agreement with
experiment is found.