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Can you make more light by using mirrors?
- Gavin (age 8)
No. Mirrors are good at changing the direction of light, but they canít make it. (Ok, if you heated them up really hot in a really hot fire, they would start to glow, but thatís nothing special about mirrors. Anything will glow if you make it hot enough.)
Sometimes mirrors can make it seem like thereís more light, though. Imagine youíre holding a candle and standing in front of a mirror. The light from the candle goes out in all directions. Some of it goes right towards your eyes and some of it goes towards the mirror. When the light going towards the mirror gets there, it bounces off and some of it comes back at your eyes. So more light does reach your eyes - you see the both the light coming directly from the candle and the light that bounced off the mirror and came back at you.
But no new light has really been made - itís just been moved around so it seems like more to you. (But imagine someone standing behind the mirror! All that light that got sent back to you canít get to them. So they end up seeing less light while you see more!)
(republished on 07/29/06)
Follow-Up #1: mirrors and the Casimir force
Not really a question. I saw this question about "Can you make more light by using mirrors?" and I saw the answer and thought. Maybe it might be fun to mention about Casimir force. (I learned it from most recent colloquium)
- Philip (age 24)
Yes, it's interesting that, when quantum effects are taken into account, two mirrors close together can attract each other. One way to describe this Casimir force is through the mirrors' effects on the quantum fluctuations of the electromagnetic field in the vacuum.
So even in the absence of any light beams, energy does depend on the relative positions of different conductors and dielectrics, such as the mirrors.
We have some answer to Casimir questions, e.g. http://van.physics.illinois.edu/qa/listing.php?id=12685
(published on 01/30/12)
Follow-up on this answer.