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Color and Heat Transfer
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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Update: June 2012
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