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Color and Heat Absorption
Name: Jessica S.
Status: student
Age: 19
Location: N/A
Country: N/A
Date: 2000-2001
Question:
My question is very similar to Edward's (in the archive
http://www.newton.dep.anl.gov/askasci/gen99/gen99540.htm) about color
and heat absorption. I would like to know (in pretty basic terms if
possible) why darker colors absorb more heat. Is it only because they
do not reflect light like white?
The answer doesn't have to get very scientific, just the basic idea.
Replies:
Yu have the basic idea black objects are black because they absorb all
the visible radiation that falls on it. That light is then converted into
infrared radiation which is essentially heat.
Vince Calder
Jessica,
Actually, rather than thinking of them as absorbers of heat, darker colors are
better absorbers of light and thereby become better radiators of heat.
Consider the following:
The color of an object depends on the wavelengths of colors reflected from the
object. A red apple is red because red wavelengths in white light are
reflected and other wavelengths are absorbed. In fact, if a red apple were to be
illuminated by light that had no red wavelengths, the apple would appear
almost black.
When a black object is illuminated by white light, all wavelengths are absorbed
and none are reflected -- that's why the object appears black. I learned this
the hard way one dark night when I tried to use my flashlight locate a Black
Angus steer that had escaped our pen. All I could see when I shined the light
on the steer were two glowing eyes.
Getting back to the point; when light is absorbed by a black object, the energy
carried by the light doesn't just disappear. Rather, it raises the energy of
the object doing the absorbing. The object, in turn, releases the absorbed
energy by emitting longer wavelength, lower energy infrared (heat). This
transformation of light into heat is the key to understanding the process
because it accounts for the law of conservation of energy. Light just doesn't
disappear when it strikes a black object -- it's transformed into another kind
of radiation that is either radiated from or retained within the black
object.
The darker the object, the better its emission of heat because it is a better
absorber of light.
Regards,
ProfHoff
You have the right idea. Heat and light are two different forms of energy.
When light is absorbed by a substance the energy is usually converted to heat.
Thus, the material that absorbs the most light gains the most energy and heats
up more.
Greg Bradburn
It is really pretty simple. The reason dark colors are dark is that they
absorb light instead of reflecting it. Since light is energy, absorbing
light makes something hotter.
Richard Barrans, Ph.D.
Assistant Director, PG Research Foundation
Darien, Illinois
There are only three ways for heat to move from one place to another:
conduction, convection, radiation.
Conduction is molecules crashing into each other. If one side of a material
is hotter than the other, heat can be conducted through.
Convection is molecules carrying energy through a liquid or gas. Hot air
rising is an example of convection.
Radiation is light waves passing from one material to another. Incandescent
light bulbs emit radiation because the wire within is heated.
Color has no effect on conduction or convection. It does, however, affect
radiation. Material looks dark if it absorbs most of the light that hits
it. A material that cannot absorb radiation will reflect it to your eyes,
making the material look like the color of the light. If a surface absorbs
most of the light hitting it, the surface heats up quickly.
Kenneth Mellendorf
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