Re: Heat vs IR

[Wayde Allen <allen@xxxxxxxxxxxxxxxx>]

On Wed, 27 Nov 1996, David Shrader wrote:

> Don't quit now!! For the non technical types why don't you summarize why
> wavelengths from approximately 10^5 microns to approximately 10^2
> microns is considered infrared? My understanding is that electromagnetic
> wavelengths beyond millimeter microwaves and before the visible spectrum
> were (ca. 1960 AD) considered "infrared" (below red in the visible
> spectrum).

This is correct.  Infra is latin for "below" or "beneath".  So radiation
in the frequency band below visible light with red being the lowest
visible frequency is infra-red.  

> Heated objects, i.e. black body, radiate electromagnetic
> energy in this frequency band and the sensation of "heat" depends upon
> the absorption cross section of the absorbing material.

Sort of...  Heated objects (black bodies) radiate at ALL frequencies, not
just infrared.  The AMOUNT of radiation at any given frequency depends on
the temperature of the radiator.  The blackbody radiation curve has been
described by several people on this list as akin to a "bell curve".  This
isn't quite correct since a "bell curve" is symmetrical whereas the
radiation curve is not, but it is perhaps a useful analogy.  The important
point is that the frequency at which the curve peaks is temperature
dependent.

You can think about a hot poker, or a heating element on a stove.  When it
is just warm the radiation spectrum peaks in the radio frequency range.
As it heats up the spectrum moves higher radiating more microwave
radiation, then low infrared, middle infrared, high infrared, and finally
begins to emit a visible red glow.  If you heat it even more it will glow
yellow, and then begin to melt <grin>.

The ability for this radiation to heat anything depends on whether or not
is gets absorbed.  Reflected energy doesn't heat anything.  The surface
area of the object absorbing the energy does influence the amount of
radiant energy that is absorbed.

- Wayde Allen
  (allen@xxxxxxxxxxxxxxxx)



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Topic No. 4