Photo: ToastyKen (flickr)
You light a candle in a dark room. Now suppose a mirror is brought in. Is the room brighter?
If you say no, you may find yourself battling your own intuition. Something in us wants to say there are now two sources of light. However, if you say yes, how can a mirror make there be more light than before? A mirror doesn’t make light.
This is a great question because it forces us to rethink our immediate assumptions. As is often the case in science, the answer to the mirror puzzle is: it depends.
In this case, it depends on where you are standing in the room. If you are standing where you can see the reflection, then your experience is exactly the same as if there were two sources of light in the room. Does that mean the mirror is producing more light? No, but it does prevent the candlelight from becoming either absorbed or dispersed in that area, and retrains it directly on your eye. The result is, it almost doubles the amount of light your eye receives.
However, if you’re standing where you can’t see the mirror, the room may actually be darker than it otherwise would have been with just a candle. That’s because a mirror points light in a specific direction, in this case away from your eyes. The overall amount of light that you actually see will then be less.