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Re: [Phys-l] paint your roof?


-----Original Message-----
From: Edmiston, Mike
Sent: Wednesday, May 27, 2009 12:44 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] paint your roof?

If it's not coupling (radiating) to the air, and not coupling to your
hand, what *is* it coupling to?

I reckon that would be "outer space." I suppose the membrane has high
radiance in infrared as well as visible.

===============================================

The membrane has high emissivity at the IR wavelengths relevant to where objects near room temperature emit and high reflectivity over most of the solar spectrum, which is about 250-2500nm (TiO2 absorbs in the UV).

The high reflectivity is associated with multiple scattering due to TiO2 particles in a polymer matrix, all of which are non-absorbing between about 400nm and 2500nm.

The high emissivity is associated with the high absorptivity (via Kirchoff's Law) of both the polymer and TiO2, since both the polymer and TiO2 absorb in the longer wave IR near 10 microns.

A roof painted white and a roof painted black will have similar emissivities, so will emissively cool the roof comparably.

An object that is reflective at visual wavelengths is not necessarily reflective at IR wavelengths.

For a nice accessible discussion of many of these points, see:
What Light Through Yonder Window Breaks by Craig Bohren, chapter 7, The Greenhouse Effect Revisited.

Room temperature objects emit near 10 microns, more generally from about 5 to 25 microns:
<http://phet.colorado.edu/simulations/sims.php?sim=Blackbody_Spectrum>

These objects emit to outer space, via atmospheric windows that exist between 5 and 25 microns:
<http://earthobservatory.nasa.gov/Features/RemoteSensing/remote_04.php>

Larry Woolf
General Atomics