Would a spherical mirror trap light forever?

03 September 2013

Question

If you could make a perfectly spherical mirror, with the mirror on the inside, and you put a photon of light inside, would it bounce around forever?

Answer

Dominic - So, you've got a perfectly spherical mirror, you've got a photon in the middle of this. I guess whatever direction that photon goes in, it will always hit the mirror and bounce off. The problem is going to making this 100% reflective mirror because any mirror that you make, it's got some small percentage chance that the light, rather than bouncing off that mirror, will hit a piece of dirt or just get absorbed as heat into that mirror. You know, for a piece of aluminium foil, about 70% of the light that hits that is reflected and if you make a really high class astronomical telescope mirror, you can get up to about 99% chance. But of course, this photon bouncing around at a speed of light it's bouncing off so many times, but eventually, it's going to get absorbed and just turn into heat.

Chris - So, is that a no.

Dominic - I think that's a no.

Vincent - Could I just ask one thing? If you could somehow associate this with the sort of solar panels, could it potentially provide energy forever?

Dominic - Well, of course unfortunately, to get energy out of sunlight, the solar panel has to absorb that sunlight and turn it into electrical energy. So in practice, in the process of absorbing that energy, that light has been destroyed. You can't use it again.

Chris - We also want solar panels that are as unreflective as possible so that they absorb as much energy and give away as much back out. That's why they're black, isn't it?

Dominic - Yeah, you want them black absorbing everything that hits them.

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