Subject: Where does light go?

I have always conceived of light dissipating over a distance as merely 
a factor of set amount of light having to fill an ever expanding volume.  
Then it struck me the other day that it must really dissipate.

What if you built a completely enclosed room covered with mirrors.  
Inside you had a light bulb. You turned it on then turned it off.  
Would the room remain filled with light?  If not (which I assume is the 
answer) where did the light go?

If the answer is that mirrors are not 100% reflective, what if you could 
build a room with 100% reflective walls?  Would the light just keep 
bouncing around?  And if you could do that, and you left the light on would 
the room keep getting brighter and brighter?

This question struck me as I was building a closet, with a light and mirror 
inside.

Re: Where does light go?