If you had a cube whose inside walls were perfect mirrors and you shined a light through a tiny little hole in one of the walls, what would happen?

Wouldn't the mirrors keep doubling the amount of light in the cube?

Would the cube heat up until it melted?

If you turned the light off, would the light still remain in the cube bouncing around forever?

If you threw the cube and it shattered, would a giant flash of light appear?

Well, keep in mind that the mirror is only reflecting the light, not magnifying it, so it isn't going to get brighter bouncing back and forth amongst other mirrors, nor is it going to perpetuate the light; as soon as the light stops going in, the light stops reflecting. If the cube broke it wouldn't release any huge flash; again you are only dealing with a reflection of the original amount of light going in.

You cant get more energy coming out of the cube than you have going in; it is a fundamental law.

But I've shined a flashlight in a dark hallway and it gets brighter when I shine the light at a mirror.

Where do the photons in the closed, perfectly mirrored container escape to when the light is turned off? I understand that photons aren't doubling (but merely reflecting) when they hit the mirrors but it seems like they shouldn't disappear when the light is turned off. I would think they would be bouncing around forever until they encountered a non-reflective surface and if the container was constructed in such a way that the hole where the lightsource was shining into the container was itself reflective then won't the container fill up with photons? And what happens as the photons continue to flow into the container? Do the photons themselves interfere with each other and act as the non-reflective surfaces?


I thought energy was a property of mass and photons had no mass.

Atoms are constantly changing energy levels. When an electron descends into a lower orbit around the nucleus, the excess energy is released in the form of a photon. When an electron wants to ascend to a higher orbit, it needs extra energy to "jump up". When a photon floats around, the electron can absorb this photon to use it to achieve a higher energy level. So eventually all photons in the room would be absorbed by surrounding electrons.
Also, a beam of light doesn't get brighter when it is reflected, because photons also have wave like properties. This means that the reflected beam cancels out the original beam (in other words, when it is reflected, it can travel into a different direction, but only at it's original strength). And then there is the problem that even a mirror isn't a very smooth surface on an atomic level, which would mean that some photons would be scattered into different directions.

At least that's how I remember it from my chemistry classes

There are problems with what everyone has said so far - the best way to put it is this:

A mirror reflects light at the same angle that it comes in at. It in no way creates more light, it just reflects the vast majority of the light that falls on it and does not scatter it very much, so you can see an image. It of course does not reflect ALL the incoming light, just a high proportion. Hence it absorbs a tiny fraction. If you ever stand between 2 perfectly aligned mirrors you will see what i mean, each further and further away image is darker and hazier.

Your mirror box would go dark faster than can be measured as soon as the incoming light was turned off - light moves fast enough that it will hit the mirrors back and forth enough times to be totally absorbed in less time than can be easily measured.

The only way to continue the "light show" would be to have the flashlight or the instrument of choice continue shining light into the mirrors. Otherwise, the energy would just expend.

I am imagining a hypothetical perfect reflective box into which a light is continuously shined.

Then it would probably expel so much energy over time that it alone would raise the temperature of the Earth by a couple thousand of degrees. ><

Well, since there would be a hole in the box, the light would just escape through said hole. If, hypothetically, there was no hole and a source of light was in the box, then really nothing exciting would happen. Since a photon (according to what you say, I'm not 100% sure) has no energy, then no heat would be generated. If the box was broken, there would probably be a slight flash as all of the light escapes, but probably nothing we would be able to see.