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Name:  Agnes L.
Status:  student
Age:  15
Location: N/A
Country: N/A
Date: 2000-2001


Question:
Absorption. My question is simply an extention of Edward's in the archive at: www.newton.dep.anl.gov/askasci/gen99/gen99540.htm About the same number a wavelengths are being absorbed in each case (because red is created when all wavelengths are being absorbed but red, and blue is created when are wavelengths are being absorbed but blue). If blue photons contain more energy than red photons then why does a blue object heat up quicker than a red object?


Replies:
Agnes,

You don't describe the experiment or give evidence for your claim. In fact, this would take a pretty elaborate experiment in which you know the same numbers of photons are being absorbed -- or you would have to account for the differences in the number of photons absorbed.

If you were to perform this experiment with adequate care I think you would find that the red material would heat faster than the blue material -- hence, red is considered a warm color and blue is a cool color.

Greg


I am not sure that it is generally true that blue objects heat up quicker than red objects, because the rate of heating depends on a lot of variables in addition to color, for example, reflectivity or "shiny-ness", heat capacity, thermal conductivity and so on.

A very important property of an object that has a large influence on its heating rate is the facility of the object to convert visible (or ultraviolet) light into infrared radiation. It is the infrared that ultimately dictates the efficiency of heat generation.

Vince Calder


Agnes,

Although a blue photon has more energy than a red photon, our sun emits more red photons than blue photons. This is why our sun looks yellow: yellow is low frequency in the visible spectrum. Also, absorption does not STOP at the ends of the visible spectrum. A red object can usually absorb far into the infrared range. A blue object can often absorb far into the ultraviolet range. Most of the sun's infrared light reaches the surface of the earth. Most of the ultraviolet light is stopped by the atmosphere: ozone. In many cases, a red object gets more energy than a blue object when exposed to sunlight.

Dr. Ken Mellendorf



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