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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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