Why does it go dark when we turn the lights off?

15 November 2016

Question

Consider a room lined with mirrors with no windows and the lights off. You stand in the middle of the room. It's dark. You turn on the lights, the room lights up, then turn them off. Why does it go dark again? Why doesn't the light from the lights just infinitely bounce around the room, keeping the room lit by the infinite reflection of the lights?

Chris Smith put this to physicist Andrew Norton from the Open University...

Andrew - This is a great question - I love this. The thing you've got to realise is that mirrors aren't 100 percent efficient. A typical mirror will only reflect 90 percent of the light that falls on it. You can get better quality ones that will reflect more. But say your mirror reflects 90 percent of the light, that means 10 percent of the light is absorbed on every reflection, and that heats the mirror up and that sort of thing.

So, if you've got to reduce the amount of light by one part in a billion of what you started with, that will only take 200 reflections. So the light bounces backwards and forwards 200 times, and you're already down to less than one part in a billion of the light you started with.

Now if you've got a typical sized room like this studio we're in - it's a few meters across. The light bouncing backwards and forwards 200 times would take maybe 2 microseconds, 2 millionths of a second. So, within 2 millionths of a second, the light bounce backwards and forwards 2 hundred times and you're down to one part in a billion. That's why it gets dark.