4. Mirrors and the law of reflection

Consider, on an atomic level, how a mirror works. Incoming light hits an electron on the mirror's surface. The electron absorbs it and immediately emits it.

Why, then, does the outgoing photon have the same angle as the incoming one? [This is essential for a mirror's function, otherwise an observer would only see a blur of color like in the last example.]

The reason for this is the same as why unobstructed light generally travels in straight lines. Photons traveling longer paths, such as A-N-B or A-P-B at right produce probability amplitudes in widely varying directions. However, paths near A-O-B would produce amplitudes in roughly the same direction. Probability is the square of resultant amplitude length, so these paths are the ones where most of the light will travel from A to B.

As these paths occur where , QED implies the "law" of reflection and thus allows billions of people around the world to comb their own hair.

If you were able to discard parts of the mirror except those that have arrows pointing a certain direction, you can "circumvent" this law and allow light to reflect at numerous locations. This type of mirror is possible - it is a diffraction grating.

Diffraction gratings, however, are of little use when flossing.

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