Atoms of Light Behaving as Waves

We have established that light has the properties of a wave and discussed the evidence in detail. It spreads out like a wave, can bend around corners like a wave, and exhibits the properties of diffraction and interference. It must be a wave.

Later we described particle properties of light. The light quantum behaves like a bullet. It can knock electrons out of the surface of a metal in the photoelectric effect, and bounce off electrons in the Compton effect; it has energy and momentum. It must be a particle.

Now that we have got used to the fact that light sometimes looks like a wave and sometimes like a particle, we meet something even more mysterious. We consider what happens when we use a very dim light source so that the photons may be separated from one another by as much as one kilometre. We are definitely dealing with individual photons, and yet somehow they have not forgotten their pedigree of a wave! When we send them through apparatus such as Young's slits, we still see interference! No photons land where there had been a dark fringe, when we had studied interference with a brighter source, even though this time two photons are never together so that they might possibly interfere. Somehow the solitary photon behaves as if it had gone through both slits at the same time! The photon is definitely an isolated particle, yet it behaves as a wave!

As if this were not strange enough, we can arrange things in such a way that the alternative paths of the photon are not close together, as they are in Young's slits but separated by an unlimited distance. Now we discover something which defies logic. Let us say that the photon has two possible paths, A and B. When we close the alternative route B, miles away, it somehow has an effect on the photon which is travelling along route A. This is action at a distance, even more mysterious than that due to electric or gravitational forces!

We finish the chapter with a glimpse of quantum electrodynamics (QED), a theory which deals with the most fundamental behaviour of light and matter.

14.1 Photons one at a time

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