# Trapped Light

## Main Question or Discussion Point

Hi. There's something I don't understand.. and it might be a silly question, but I want an answer.

Light is formed of photons, and photons are reflected off mirrors. If you reflect photons off one mirror onto another mirror, then have that second mirror reflect back onto the first.... why don't the photons get 'trapped' forever bouncing back and forth? Considering they took days, months, thousands of years to arrive here from where they were generated.

Confused.

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That sounds like a light clock. It would be trapped, in an ideal mirror configuration.

Drakkith
Staff Emeritus
Hi. There's something I don't understand.. and it might be a silly question, but I want an answer.

Light is formed of photons, and photons are reflected off mirrors. If you reflect photons off one mirror onto another mirror, then have that second mirror reflect back onto the first.... why don't the photons get 'trapped' forever bouncing back and forth? Considering they took days, months, thousands of years to arrive here from where they were generated.

Confused.
Given perfect mirrors you could make that happen, however we can't make 100% perfect mirrors, so there will always be some photons being lost.

What does the time it took a photon to arrive have to do with this by the way?

Even if mirrors are 99.999% reflective, there are c/L bounces every second. If the mirrors are 10 cm apart then you have 3 billion bounces every second, so all the energy will be lost very very quickly. That's why this is never observed.

Even if mirrors are 99.999% reflective, there are c/L bounces every second. If the mirrors are 10 cm apart then you have 3 billion bounces every second, so all the energy will be lost very very quickly. That's why this is never observed.
I am assuming this also applies to a closed mirrored box correct?

Andy Resnick
Hi. There's something I don't understand.. and it might be a silly question, but I want an answer.

Light is formed of photons, and photons are reflected off mirrors. If you reflect photons off one mirror onto another mirror, then have that second mirror reflect back onto the first.... why don't the photons get 'trapped' forever bouncing back and forth? Considering they took days, months, thousands of years to arrive here from where they were generated.

Confused.
Something similar can happen for highly scattering disordered materials:

http://www.complexphotonics.org/andersonlocalization.html

It would be trapped, in an ideal mirror configuration.

Drakkith said:
Given perfect mirrors you could make that happen, however we can't make 100% perfect mirrors, so there will always be some photons being lost.

So if we could make an 'ideal' '100%' perfect mirror configuration, we could trap light indefinitely?

Drakkith said:
What does the time it took a photon to arrive have to do with this by the way?
I was just wondering if the energy would be used up in some way, like degrading over time.

Curl said:
Even if mirrors are 99.999% reflective, there are c/L bounces every second. If the mirrors are 10 cm apart then you have 3 billion bounces every second, so all the energy will be lost very very quickly. That's why this is never observed.
Mathematicians always tell me I am stupid for differentiating between 99.999% and 100%! :D I shall remember this example for the next time.

Assuming we could get a 100% reflection, and the photons were thereby trapped, how long would they remain so? Indefinitely?

Thanks all for helping! :)

ZapperZ
Staff Emeritus
At some point, it is really meaningless to talk about something that doesn't occur.

There are better ways to http://physicsworld.com/cws/article/news/27007" [Broken] that have been shown empricially. There is no need to waste time on things like this.

Zz.

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At some point, it is really meaningless to talk about something that doesn't occur.

There are better ways to http://physicsworld.com/cws/article/news/27007" [Broken] that have been shown empricially. There is no need to waste time on things like this.

Zz.
You are, of course, entitled to not waste your time and thereby to not post in a thread that you consider to be worthless.

For me, I am interested in learning what is possible, not just what actually does occur.

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Drakkith
Staff Emeritus
Assuming we could design perfect mirrors the light would remain trapped effectively forever. However, we cannot make perfect mirrors, it simply isn't possible for us.

Andy Resnick
Assuming we could design perfect mirrors the light would remain trapped effectively forever. However, we cannot make perfect mirrors, it simply isn't possible for us.
That's not exactly true- mirrors based on total internal reflection are perfect reflectors. See, for example, optical fibers.

Drakkith
Staff Emeritus
That's not exactly true- mirrors based on total internal reflection are perfect reflectors. See, for example, optical fibers.
If so, could you design one that would forever reflect light back and forth as in the OP's example?

I think optical fibers aren't 100% reflective either. There is always a probability that some of the light is absorbed (thus increase entropy).

Andy Resnick
I'm not claiming $\infty$ is physically attainable.