system
Given the following post, respond as if you're commenting on Reddit:
human
Is it possible to trap light in a hollow ball whose inside is made out of mirror?

Say you have a ball which can be opened. Inside the ball is purely mirror. You put a flashlight that is turned on inside and you close the lid for a million years. The flashlight will have run out of battery long before then.

Within the ball, will the light just keep bouncing forever?

Now after a million years, someone or something opens that ball with an internal mirror. Will a flash of light occur when the ball is opened?
gpt
Mirrors don't reflect light perfectly. With every "bounce", part of the light is absorbed instead of reflected and the reflection loses some of its intensity.

Regular everyday mirrors will top out at about 90% reflectivity. That means that 10% of the light intensity is lost with each bounce. If the distance between each bounce is 1 meter, that means it takes about 3 ns (nanoseconds) for 1 bounce. After 50 bounces (150 ns), only 0.5% of the original intensity remains.

It's possible to fabricate a high reflective coating that does much better however, going up to 99.99% or even higher, but it comes with some limitations. One of them is that these coatings typically only work on a relatively narrow range of wavelengths. They can also be sensitive to the angle of incidence of the incoming light.

But lets forget about that and see what happens with a 99.99% reflective coating. Only 0.01% is lost per bounce. However, we find that after only 50,000 bounces (= 150 microseconds), only 0.7% of the original light remains when the distance traveled between each bounce is 1 meter.

Lets make our mirror even better and shoot for 99.9999% reflectivity (I have no idea if this is possible) and increase the size of our mirror-box so that there's 1 km between bounces. What now?

After 5,000,000 bounces, only 0.7% of the light remains. How long does that take? At 1 km distance, each bounce takes about 3 microseconds. So the 5,000,000 bounces take a total of 15 seconds.

So even in the very unrealistic example of a 99.9999% perfect mirror with a 1 km path length, you'd still have lost almost all of the light in well under a minute. This is ignoring the problem of losing light intensity to air and dust in the interior (you'd have to make the interior of the mirror box a vacuum) and the fact that the flashlight itself is in the way of some of the light and will cause additional loss of intensity as light strikes it.