Re: Where does light go?

The idea of 100% reflective mirrors is not that farfetched.
If you were to construct such a closet, it would indeed "fill
up" with light, since there is nothing (aside from the light
bulb, but we'll dodge that for now) that can absorb the light.
Yes, the room would get brighter and brighter.  You could
never actually see this going on, since opening the room up
would release the accumulated energy.

Light is another means of storing energy.  If you prevent the
energy from tranfering to another form (heat, electricity,
chemical, etc.) then it will hang around, untill you allow it 
to make the change.