I did not mean to insult either you or biology teachers. My
reference to Feynman is appropriate to *some* high school
biology teachers, but I'd bet the fraction is less than 5%.
Your explanation of why the sky is blue is fine, but the
question dealt with sunsets, and my answer pertains to
sunsets. Surely not all sunsets in Utah are red. Do you not
have sunsets too bright to look at until the last sliver of
the Sun's disk is all that is left? Is it ever red in that
circumstance? I'm asking a real world question, and I think
everyone should do some careful observation of the mundane
phenomenon of sunset.
There is an underlying question here which is of some
considerable importance to physics teachers. It is the
phenomenon of color. Feynman devotes considerable space to
color in his Lectures (Chs. 35 & 36) but most orthodox
texts either avoid the topic or else say incorrect things
about color which seem to dismiss it as so simple and
evident as to be unworthy of attention. The Sun at sunset
in clear air, if it can be said to have color at all, is
yellowish-white. The phenomenon of color constancy will
prevent one from seeing its color subjectively in, say, a
white paper illuminated by the setting sun, but of course
that is not the manner in which a sunset is observed and
said to be red anyway. White paper will look white even
when illuminated by a very red setting sun!
I do discuss color phenomena at some length with a group
of physics majors I meet with weekly in a noncredit, more
social setting we call the "Monday Afternoon Physics Tea
Party" by tradition. Color is a worthwhile topic that
teachers of physics could explore enjoyably starting with
those chapters in Feynman.
In summary:
My apologies to Dan and any biology teachers I insulted.
The insults were unintentional
When one observes a red sun at sunset, the color is due to
preferential scattering of shorter wavelength light from
the Sun by particles in the atmosphere. The scattering due
to air molecules which account for the blue color of the
clear sky is much smaller.
I hope this last paragraph clarifies my initial posting, q.v.