Re: Glacier Color

[Leigh Palmer <palmer@SFU.CA>]

> According to Hecht in his Optics text, almost any time you see blue in
> the animal world (blue eyes, blue jay's feathers, even a baboon's blue
> buttocks) it is due to Rayleigh scattering.

How 'bout the morpho butterfly, or a mallard's head when viewed at the
correct angle to see blue? Those are both interference colors, and
many more examples of interference colors (even into the ultraviolet)
are to be seen in the animal kingdom. There may also be some blue
pigments in some species; I expect there are. Hecht's statement as
reported here is excessive.

It is an injustice to call this phenomenon "Rayleigh scattering" since
it was first demonstrated by John Tyndall. Both Tyndall and Rayleigh
initially thought that the phenomenon was due solely to particles
suspended in the air or gas, and Rayleigh only changed his mind after
Tyndall's death. Biologists refer to mechanism of the the blue to
which Hecht refers as "Tyndall scattering", and it is in these cases
always particles which are responsible. "Rayleigh scattering" is now
usually taken to be synonymous with molecular scattering according to
Jürgen Meyer-Arendt's textbook "Introduction to Classical and Modern
Optics". Jenkins & White similarly recognize the correct discoverers.
Hecht does not follow this convention. Tyndall does not appear in the
index to the earlier book by Hecht & Zajac (my Hecht is at home).

Leigh