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Photon Tennis

Here's a perplexing question involving an everyday object, how can light bounce off a mirror?

Light on Hand from Reflection

Photo: hlkljgk (flickr)

How can light possibly bounce off a mirror??

Here’s a perplexing question involving an everyday object, how can light bounce off a mirror?

Imagine first a tennis ball bouncing off a wall. It doesn’t maintain the same speed at all times. It has one speed while approaching the wall, then as it hits the wall, the speed rapidly decreases as the ball distorts. At this point, the ball briefly comes to a complete stop. Only a moment later it begins to recoil and pick up speed, flying in the other direction.

None of this can be true for light, because an individual particle of light, called a photon, must maintain the same speed both going into the mirror and coming back out. It can’t rebound at less than its original speed, or ever stop. That’s the rule with photons.

Now, we come back to the original question, how can light bounce off a mirror?

The answer is that it doesn’t. When an individual photon hits a mirror, it does not bounce. Instead, it is absorbed into one of the atoms that make up the mirror. For a tiny amount of time, that particular atom has more energy than it did before, and it spits out that same amount of energy, at exactly the same frequency. The two are indistinguishable, but the original photon never had to slow down because it was absorbed and another one emitted in its place.

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