Does light bounce off the walls of a room?

11 July 2013


Hello from Seattle!

I absolutely love your podcasts -- I listen to them religiously. Keep fighting the good fight -- you are a beacon of science literacy for the entire world!

What happens to light when it hits its target -- whether from a star or a flashlight? I know part of the light is converted to heat, and part reflects. But the light that reflects... does it reflect more than once? Is there a limit? For example, if I turn off the light in my house, it is dark instantly -- why is that? Shouldn't the light be bouncing around all over the place? Seems like there'd be some residual energy we'd be able to perceive? I'm not talking about mirrors per se -- just "normal" interaction with the environment. Hope that made sense. :)

Brian Karsh


Dominic - Well, I think the important thing to remember here is that light travels incredibly quickly at about 300,000 km every second. So, that's far enough that it gets from the sun to the Earth in about 8 minutes. Light certainly does scatter off all sorts of different surfaces, but when you got light on your ceiling, that will be illuminating your walls, but your walls appear bright because the light is scattering off those walls, and they'll for example preferentially scatter some colours more than others. So, the walls in the studio look blue because they're scattering the blue light from the lights above us, but they're not scattering the red light. But they're doing that so incredibly quickly that when you turn the lights out, within about 100 millionth of a second, all of that light has been absorbed into those walls as heat and so, the room is dark. So in fact, when you turn the lights out, you'll see the light gradually fades because the elements in those lights which are gradually cooling down. And that's a much slower process in how it takes for light to travel around the room. Chris - So, if I did have a camera that was sufficiently fast, I would effectively see what's being suggested, but because our eyes aren't sufficiently fast, it appears to us as though it goes dark instantly.

Dominic - Yes and I think about 18 months ago on this programme, Dave Ansell had a news story where somebody had made a camera that could take millions of frames a second and by doing that, you could actually see light ricocheting around the room.

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