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From: ems@amdahl.UUCP (E. Michael Smith)
Newsgroups: net.cooks,net.flame
Subject: Re: Truth in Advertising?
Message-ID: <518@amdahl.UUCP>
Date: Thu, 15-Nov-84 15:52:36 EST
Article-I.D.: amdahl.518
Posted: Thu Nov 15 15:52:36 1984
Date-Received: Sat, 17-Nov-84 08:00:50 EST
References: <2559@dartvax.UUCP>, <642@watdcsu.UUCP> <216@pyuxt.UUCP>
Organization: Amdahl Corp, Sunnyvale CA
Lines: 41

> >I still can't believe the ignorance prevailing in journalism today!
> >For someone to draw the conclusion that since black cars get hot in
> >the sun, then black pots must radiate lots of waste heat is
> >incredulous!  This person should have at least consulted a basic
> >physics text.
> 
> >On the other hand, an object of colour x can only radiate light (read:
> >heat) of colour x (i.e. that wavelength).  Therefore, a 100% black pot
> >will radiate *no heat* at all.  It will be warm to the touch, though.
> 
> Is this why my physics book used to talk of "black body radiation"
> with the same reverence that it used for "perfectly elastic spheres."
> I think black is the *best* radiator.
> 
> 		Marcus Hand	(pyuxt!marcus)

Marcus is right.  The mistake in the first argument is this:  the
conclusion that a body of color x radiates only x therefore black,
having no color doesn't radiate.  The fact is that black is *all*
colors and thus radiates *all* colors.  Absorption and radiation
form a pair.  Reflection and radiation do not.  The color *reflected*
is not radiated well.  (Think of the color you see as the color
the object *is not*.  That is, the one it reflected away because it
could not absorb it. Silvery surfaces reflect all colors well and
do not radiate well at all.  Of large importance, though, is the
reflectivity/absorptivity in the infrared range which cannot be
judged by eye.  Your silvery pot may be 'black' in the IR while
the black pot may be 'shiney' in the IR.

An interesting example of this is the green garden hose phenomenon.
One green hose gets *quite* hot in the sun, another does not. Why?
Both are absorbing large quantities of radiation.  One is radiating
it away in the IR, the other (a different plastic with different IR
radiation abilities) cannot radiate as well and so becomes quite hot.

-- 

E. Michael Smith  ...!{hplabs,ihnp4,amd,nsc}!amdahl!ems

The opinions expressed by me are not necessarily those of anyone.
(How can a company have an opinion, anyway...)