Using light as an energy condenser If I created a sphere with the internal surface perfectly mirrored and shone an intense light into it from a small aperture, what would happen to all the photons being accumulated? If I then at some point later opened another small aperture, could I then use those accumulated photons on a photovoltaic surface to generate electricity? So what we have is the ultimate Battery, obviously with losses creating the first photon stream (from sunlight), and losses in the photovoltaic conversion, but in between an accumulator of light energy - a battery.
 A: A few things you need to consider:
(1) You won't be able to put an arbitrary amount of light into the box -- at some point the pressure of the light inside will be greater than the pressure of the incoming light, and then no more will get in.
(2) A far bigger issue: in the real world no reflecting surface is 100% efficient, and the light is going to be bouncing a lot. If the box is one meter across, light will reflect from its sides 300 million times per second. If the mirror is 99.999999% reflective, that means that once in every 100 million bounces a photon will be absorbed rather than reflected. Which means that the light will stay in the box for something like 1/3rd of a second before being absorbed.
A: Interesting thought aside from loses on reflection as Eric pointed out. But there are a couple other problems, too.
Suppose you have a lot of light bouncing around inside. Suppose you open a door so some can get out. How do you allow only a little out at a time at a controlled rate?
One problem with batteries is they concentrate a lot of energy in a small volume. If that energy comes out slowly, it is useful. But if it comes out all at once, it is a bomb. Suppose you put lots of energy inside in the form of light. Suppose you dropped the sphere and it broke. The flash of light would burn anything nearby. Perhaps it would make a better hand grenade.
