Re: Long Prisms for homemade spectrometer

[Leigh Palmer <palmer@SFU.CA>]

> Does anybody have a supplier for long 90-degree prisms (say, 1"x1"x12")?
>
> I'm working on making a lab for students to determine the albedo of the moon
> by comparing spectra of the sun and moon.  As a secondary goal, I have this
> thought that the spectra off a reflecting body should be a bit different
> than a straight-shot spectra.  Is there any truth to this??

If there is relative motion (as in the rings of Saturn) then there
will be doppler shifts present in the reflected radiation. That's
how one demonstrates the motion of particles in the rings. Other
than that, you should see the same lines.

> I thought I could get clear of the "you'll burn your eyes out!" arguments by
> simply projecting the spectra onto a sheet of paper for the students to
> marks where the absorption lines were, hence the need for a long prism.

One doesn't simply pass sunlight through a prism to demonstrate the
Fraunhofer absorption spectrum of the Sun. That works for Newton's
demonstration of the phenomenon of colors, but for the narrow lines
one needs lenses to collimate and focus the radiation, and an
accurately aligned entrance slit as long as the desired spectrum is
wide. After all, the Sun has an intrinsic width of half a degree. I
doubt that the absorption lines will be visible in projection onto
paper in the case of moonlight*. The same collimation requirements
hold for the Moon as for the Sun, and at the resolution required to
resolve the lines you will have difficulty with signal strength.
You should try out your working solar spectrograph using a piece of
white paper reflecting sunlight as your source; you don't have to
wait for the Moon to come out. This should give you a signal six or
seven times as strong as you will get from the full Moon itself.

These considerations make it unclear why you would want to use a
prism of the sort you seek. Let us know how it all works out.

Leigh

*You may have a fighting chance looking through a ground glass. To
succeed you will surely have to use a hood of some sort to preserve
your dark adaptation in the brilliant moonlight.