Would light in a sealed mirrored box bounce about forever?

06 April 2008

Question

If I put lights in a perfectly sealed box with mirrors inside it, would the light bounce around inside for eternity?

Answer

Dave: It depends on your mirrors. Normal mirrors which you use at home actually only reflect about 80% of the light so by the time it's gone backwards and forwards a few times every time you're going to lose 20% of what you've got left. Light moves at about 300 million metres per second so it's going to bounce backwards and forwards in your box very quickly. Even the best possible mirrors we make will lose about 1% of light every time you do it. So the answer is yes but in practicality, no. Chris: What would you do with it if you could and how would you know it was in there? If you tried to detect it you'd soak it up and you can never do that, could you?Dave: You could put some light in there and come back two months later and see if it's still there but yeah, it's not especially useful.

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