# A light conservation.February 18, 2009 3:42 PM   Subscribe

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 (5 answers total) 1 user marked this as a favorite

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

No. Light diffuses over space (its not as strong 100 feet away as it is 1 foot away from the source). Over shorter distances, it might be harder to spot.

If you want to talk about a waste...most of the energy that goes into powering a traditional light bulb is wasted as HEAT, not light.

THAT is a waste.

If you really are worried about light pollution (or seepage), you might want to get one of those switches where you can dial down the power of the lamp.

Good luck.
posted by hal_c_on at 3:54 PM on February 18, 2009

hal_c_on mentions dimmers.

First, dimmers don't really work for CFLs. So you'd need to use inefficient old edison bulbs. Secondly, lots of dimmers are just potentiometers, a resistor you could control the resistance of. These convert some of the electricity to heat, leaving less for the light bulb.

Better dimmers actually have a really high frequency switch that turns the bulb on and off many times per second, and more dim is the same thing as being on less. It's so fast (and the tungsten takes a while to cool down anyways) and it looks smooth.

Moral of the story: Since we care about efficiency, we use CFLs. The only situation where you use dimmers is either Bad or Extra Bad.

In our LED-lit future we can control the brightness though.
posted by floam at 6:20 PM on February 18, 2009

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

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