The photograph below shows how a double slit interference pattern is built photon by photon. We are forced to say that when the light hits the screen, the light is a stream of photons. But is it a stream of photons when it encounters the slits? If so, we run into some hard questions to answer, like which slit does a photon go through or how does a photon "know" where to go once it leaves a slit. The conventional thing to do is to avoid this quandary by saying that the light is a wave when it hits the slits. Light exhibits a wave-particle duality. Sometimes we must model it as a wave and other times as a stream of particles (photons).

Photograph from Newton to Einstein: The Trail of Light by Ralph Baierlein (Cambridge University Press, 1992).


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