Re: Sunsets

[Leigh Palmer <palmer@SFU.CA>]

Red sunsets are usually due to scattering of light by atmospheric
particulates: dust or aerosols. These particles scatter shorter
wavelength light more strongly than longer wavelength light. When
the Sun is observed through a long path containing particles A
much greater fraction of the shorter wavelength light is scattered
out than of the longer wavelength light, leaving the image of the
Sun both dimmer and redder than it would be seen through a less
strongly scattering atmospheric path.

I spent a couple of days on Galianao Island cycling and hiking
with John Mallinckrodt. The particulate scattering can be observed
well before sunset (or after sunrise) by looking at successive
mountain ridges in the distance. A highly transparent atmosphere
will make distant mountains appear nearer than they do when seen
through a scattering atmosphere. In the latter case the mountains
look a bit bluish as sunlight is preferentially scattered toward
you with a dark background (we have trees on our mountains). The
effect increases as one observes more distant mountains. The sky
is also blue, but particulate scattering plays a variable role in
that case. (In LA the sky may even be white!) When one looks at
a transparent sky from a great altitude it may appear darker.

I think that the mathematical theories would not be of use to your
colleague, and the sort of descriptive explanation I've given,
reworded in your own language, will probably be more effective
than any source in the literature that I know of. For an example
of what I mean see section 32-5 in volume I of the Feynman Lectures.

Leigh

> A biology teacher asked me a question about sunsets today that got my
> juices flowing on this topic again.  I have been looking for a truly
> satisfying answer to the question of why red light dominates during
> sunsets or for that matter why blue dominates most of the rest of the
> time.  Can anyone provide more than a hand wave or direct me to a good
> source?
>
> --
> Cliff Parker
>
> Never express yourself more clearly than you can think.  --  Niels Bohr