Black Body (Re)radiation
Name: Dallas T.
Date: Saturday, April 20, 2002 8:17:44 PM
I understand why a black body absorbs heat quicker than a
white body, but why does a black body emit heat quicker than a white body?
A black body absorbs radiation heat (photons of light) better than a white
object. When light hits the black object, the energy gets absorbed and held
long enough to distribute through the material. When light hits a white
object, it gets re-emitted almost immediately. The energy of the light does
not have enough time to be distributed.
For a black object, changing heat to light is easier for the same reason.
Energy transferred to a surface atom can be held long enough for more and
more energy to build up. Soon there is enough energy in the atom to produce
light. You may not see the light because it may be infrared. A surface
molecule of the white material cannot hold the energy long enough to build a
photon of light. It may occur now and then, but not nearly as often as for
the black material. Thus, a black material can absorb and emit energy
faster than the white material.
Dr. Ken Mellendorf
Illinois Central College
Suppose you put a black body and a white body in a room and hit them
both with a light (shine a flashlight,whatever). They both begin to
absorb the light, and warm up, maybe at different rates of temperature
rise, but both warm up. But they cannot warm up forever, they would go to
infinite temperature. As they each get hotter, they each radiate heat
back out. Eventually they each must come to some (equal) equilibrium
temperature. But they each continue to absorb at the different rates, so
they must radiate at different rates, each precisely balanced so that in
= out. White rate absorbed is not black rate absorbed, but is the same
as white rate radiated.
I think that there is an ambiguous use of the term "blackbody" vs.
One word vs. two words. A blackbody is a body that emits radiation according
to Plank's Law. The distribution of radiation within a blackbody (think of
it as a small hole in a large can) depends only upon the temperature of the
body and not on its composition or "color". When you refer to a "black vs.
white body" you mean what is the distribution of wavelengths of light that
are REFLECTED by an object. A white body reflects all wavelengths of visible
light equally. So if you shine a red light onto a "white" body it appears
red. If you shine a blue light onto a "white" body it appears blue. A
"black_body" reflects no wavelengths of visible light. It is not clear to me
exactly what you mean by the term "quicker" since the change in temperature
of an object depends on many factors -- heat capacity of the object,
convection of air in contact with the body etc.
Use www.google.com to search the term "blackbody" (one word) and you will
find numerous sites discussing the subject, at all levels of sophistication,
and many with very pretty graphics.
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Update: June 2012