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Name:  Katy M.
Status:  student
Age:  20s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
Can you please settle a dispute about the effect of color on heat transfer IN THE DARK? I say that since dark colors absorb more light and that's what results in their increased heat, if they only way they are receiving heat is though means other than light, the color will make no difference.

My BOYFRIEND, on the other hand, thinks that a baked potato will cook more quicky with the shiny side out, and that you'll sleep more warmly in a sleeping bag with the shiny lining, and that this is why house insulation has a shiny coating on it. (of course not only is the "color makes a difference in the dark" logic wrong in my opinion, but it's ALSO backwards because in the light it'd be the dull, not the shiny, objects that would absorb more heat, right? In addition to shiny objects in the dark, could you also address different colored objects in the dark?


Replies:
You are considering the absorption of visible light. Energy that is absorbed will cause the temperature to increase.

The other side of the coin is how quickly something loses heat, -- i.e., energy emission. Anything we can do to slow or reduce the energy emission will result in maintaining a warm temperature for a longer time. Emission of energy occurs in the infrared so the "color" perceived in the visible doesn't really tell us much about the infrared emission characteristics. However, things that reflect infrared radiation do tend to be "shiny" (highly reflective) in the visible. A sleeping bag with a shiny coating (assuming it is reflective in the infrared) should keep you warmer at night and house insulation does have a shiny coating to keep heat from radiating through it (thus keeping heat inside in the winter and outside in the summer). Using this argument the baked potato should cook more quickly with the shiny side in.

Absorption and emission are linked to each other. Anything we do to increase the absorption of radiation will also increase the emission of radiation (for a given wavelength of light). To effectively use solar energy for heating, the key is to make materials that absorb lots of visible light where most of the solar energy is concentrated but absorbs (and thus emits) very little infrared radiation.

Greg Bradburn



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