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Re: hoaxes



Hewitt: "A cloud is composed of a variety of droplet sizes. The tiniest
scatter blue, slightly larger scatter green, and still larger scatter reds.
The overall result is a white cloud."

This is mistake, not a hoax.

(He also is wrong when he says that clouds are dark because they are
absorbing more light)

Water droplets of clouds range between 1 and 30 microns, 10 microns is
typical:
http://www.met.ed.ac.uk/~ben/factors/node10.html

For particles significantly larger than the wavelength of visible light, the
scattering of visible light is relatively independent of wavelength. In
addition, clouds are optically thick - the light is scattered many times
before it reaches your eye.

Simple experiment - add a few drops of milk to water. When you add enough
drops of milk to turn the material a milky white, there is sufficient
multiple scattering for all of the visible light to be completely scattered.

For a fine explanation (where I learned about this), see

"Clouds in a Glass of Beer" by Craig Bohren, pages 104-119 and especially
pages 107-109.
"A single cloud droplet is a nonselective scatterer of visible light. This
is sufficient to make a cloud of such droplets white upon illumination by
white light. But it is not necessary. An optically thick collection of
negligibly absorbing particles (or molecules) is white (under illumination
by white light) regardless of the wavelength dependence of scattering."

Milk, salt, sand, sugar are white because of multiple scattering in an
optically thick medium by weakly absorbing particles.

See also:

"At What Optical Thickness Does a Cloud Completely Obscure the Sun?, Craig
F. Bohren et al, Journal of Atmospheric Sciences volume 52, No. 8, 15 April
1995, pp. 1257-1259.

"Multiple scattering of light and some of its observable consequences,"
Craig F. Bohren, Am. J. Phys. volume 55, No. 6, June 1987, pp. 524-533.

White pigment primer (TiO2)
<http://www.titanium.dupont.com/TTPORTAL/EN/Products/Literature/CO_B_H_65969
_Coatings_Brochure.pdf>

Larry Woolf;General Atomics;San Diego CA
92121;Ph:858-526-8575;FAX:858-526-8568; www.ga.com; www.sci-ed-ga.org

-----Original Message-----
From: Tim O'Donnell
Sent: Friday, April 11, 2003 1:36 PM
Subject: Re: hoaxes


Note that there are many examples of errors in
a. texts (Hewitt's discussion of why clouds are white in
his=20 Conceptual Physics 8th edition, p. 481) [snip]

Do you think this is a hoax or just an error?
What is the reason clouds are white?