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Black Absorption and Radiation
Name: J. P.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 2001-2002
Question:
What historical evidence is there behind the fact that
black surfaces absorb and radiate heat better than shiny ones. What
practicals were carried out in order to prove the fact. Could someone
also explain why black surfaces can absorb AND radiate better than shiny
surface - KEEP IT SIMPLE PLEASE - this is for Year 9 pupils - 13/14 year olds.
Replies:
There are several things going on here that need to be unraveled.
Three processes occur when a surface is exposed to VISIBLE light --
absorption, scattering, and specular reflection. A surface that absorbs all
the VISIBLE light (black felt or charcoal are good examples). Specular
reflection is the process whereby the angle of incidence of the incident
light (100% in the ideal case) equals the angle of reflection. An example of
course is a mirror. Scattering is similar to reflection except the surface
is rough, so that the angle of reflection takes on different values than the
angle of incidence. Such surfaces appear white in the idealized case. Where
absorption and scattering occur equally at all visible wavelenths the
surface looks more or less gray. If the surface preferentially absorbs SOME
wavelengths of visible light, but not others, the surface appears colored.
The color will be the complement of the wavelength absorbed because some
wavelengths are preferentially REMOVED from the incident light.
How HOT a surface becomes depends mainly upon the amount of infrared
radiation is absorbed by the surface. The confusion arises because the
absorption, and scattering in the NON-VISIBLE LIGHT, either infrared or
ultraviolet is DIFFERENT (usually) than it is to visible light. So, for
example, a metal may reflect a lot of visible light, but efficiently absorb
infrared wavelengths, hence the title "Cat on a Hot Tin Roof". In all of
this we have assumed that all wavelengths are present in the incident beam.
This may not be the case.
The other thing you refer to is the phenomenon called "black body
radiation", which is approximated by a large container with a small hole in
it. It is referred to as a "perfect absorber and emitter" because the
number of photons generated by the walls of the container equals the number
of photons absorbed by the walls of the container. Planck, and others showed
both theoretically and experimentally that the spectrum (distribution of
wavelengths) of the radiation depends solely on the absolute temperature,
and not the nature of the wall etc. The container looks "black" at room
temperature because we do not "see" any of the radiation, which is in the
infrared. However, if the container is heated to about 500 C and above it
will appear red then yellow, then increasingly blue as the temperature
increases.
In the real world processes that convert radiation of one wavelength to
another so the picture gets more complicated.
Vince Calder
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