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Absorption, Heat and Wavelength
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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