A light conservation.

February 18, 2009 3:42 PM

Does light (like heat) seep and weaken?

You have two 15' square rooms, no windows, painted white, with a solid wall in between and a bright light in the middle of just one. You also have two 15' square rooms, no windows, painted white, with a glass wall in between and a bright light in the middle of just one.
Would the light in the second lit room be as strong as the light in the first lit room?
I got to thinking about this because I have glass doors leading into my dining room (which is usually unlit). The room is still pretty bright from leakage, and the light even brightens up the windows leading outside. The light then goes into infinite space. Seems a waste.

posted by Pennyblack to Science & Nature (3 answers total)

Best answer: White paint has an albedo of maybe 0.75, so every time the light hits a wall, 25% of it is absorbed, and the rest is diffusely reflected around. In your second scenario a bunch of light is absorbed in the second room and doesn't make it back through the window into the first room, so the room will seem at least a little darker than if there were a white wall instead of a window into a white room.

The escaping light will just turn into a small amount of heat while bouncing around your walls. The amount of energy "wasted" by indirectly illuminating the approach to your door is pretty inconsequential.
posted by aubilenon at 3:50 PM on February 18, 2009 [1 favorite]

No, because youre only producing some many photons. For your eye to see the same brightness you need the same amount of lumens in the space. So lets say its a 60 watt bulb. Say 200 lumens per watt. That 12000 lumens split between the two spaces. The second space will look half as bright as the first space. The amount of photons bouncing around will be less and you'll see that as dark.
posted by damn dirty ape at 3:53 PM on February 18, 2009

Yes, aubilenon is correct. Everyone else is confused.
posted by alexei at 8:33 PM on February 18, 2009