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Q & A: Photon paths near a mirror

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Most recent answer: 10/22/2007
Q:
If you shine a torch at a flat mirror, some of the light is reflected straight back at the torch (ie: at right angles to the mirror). Does this mean that some of the photons had to stop completely dead before they could be relected? If so, all of their energy must have been lost, so how do they get their energy back? Is similar to bouncing a ball?
- Muhammed (age 18)
A:
Hi Muhammed,

It doesn't really make sense to ask about the histories of individual photons. It's not that that's a bad question -- it's actually a really really good one. It's just that quantum mechanics insists that answers to questions like that don't exist because you cannot list the places an individual photon's been and the times it's been there.

When doing an experiment where a photon interacts with a single atom or molecule, then a photon behaves as if it is a localized packet of energy which appears at one place and time. But to examine how a photon propagates from one place to another, one has to use the wave description of photons. When an electromagnetic wave impinges on a mirror, another electromagnetic wave is emitted by the mirror in order to satisfy the boundary conditions on the surface of the mirror that the electric field component parallel to the surface of the mirror vanish. That, and the magnetic field's component pointing into the mirror must vanish. The only solution to this including an incoming wave includes also an outgoing wave obeying the usual laws of reflection. And then the waves can be thought of as probability functions for finding individual photons.

Actually, balls "reflect" from surfaces too, and in similar ways. Macroscopic balls do have detailed histories and you can describe where they are at all times, and they do come to rest before rebounding. Microscopic objects, like individual atoms, are described better with the laws of quantum mechanics, and there the incoming and outgoing wave functions are needed to describe scattering of atoms from solid walls -- they too exhibit wave-particle duality. It is currently an area of active research to see how big a system can be and still follow simple quantum rules like a single atom, and at what sizes and other external conditions are needed for the more classical description to be appropriate. I think the current achievement is to get buckminsterfullerene molecules (round balls made up of 60 carbon atoms in a symmetrical arrangement) to interfere quantum mechanically with themselves.


(for our U.S. readers, "torch" = "flashlight")

(published on 10/22/2007)

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