🚀 go-pugleaf

On Mon, 1 May 1995, BRANDON COLBY wrote:

> Just a quick question-
>
> When I was thinking about the dispersion of a rainbow I couldn't remember
> what color was on top, violet or red and why it is? Also, what is the exact
> explanation of how a rainbow is formed.
>

In the *primary* (smallest) bow, red is on the outside, violet on the
inside. In the *secondary)* (larger) bow, the order is reversed. The sky
just inside the inner bow is brighter, due to mixed colors from droplets
which make an angle (sun to eye) which is not minimimum deviation). The
sky just outside the outer bow is brighter also, for the same reason.

The color reversal between inner and outer bow can be explained from the
fact that the rays forming that bow suffer two reflections inside the
drop, and those forming the inner bow suffer only one, hence the outer
one has a reversal of collors (but some more details, and diagrams, should
be supplied in the explanation).

The rays passing through a single prism are deviated so that violet is
deviated most. I know of no way to physically derive this fact in the
context of an introductory course (and I'd *love* to be shown wrong on
this). In a sense it seems counter-intuitive to students, since the
energy per photon is greatest for violet, and these photons are deviated
the most. Can one argue that they suffer more frequent absorptions and
re-emissions, with the accompanying time delay of each?

By the way, I just discovered that the clear plastic hexagonal shell of
BIC and Pilot point pens makes a nice prism in sunlight, and at a few
feet casts a nice spectrum about an inch across. One can also rotate the
pen and show minimum deviation easily. Cheap prism. Don't throw away your
old dispsable pens of this time. You can easily remove the ink cylinder
from these, though the work as prisms even with it in place. The prism
angle is, of course, 60 degrees.

        -- Donald