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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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