Re: Long Prisms for homemade spectrometer

[Gordon Smith <gsmith@HSC.EDU>]

--snipped from John Denker's reply--
> > I'm working on making a lab for students to determine the albedo of the
moon
> > by comparing spectra of the sun and moon.
>
> I don't understand the point.  The usual definition of albedo
>   http://www.m-w.com/cgi-bin/dictionary?va=albedo
> doesn't have any connection with the shape of the spectrum.

Ah, I misspoke.  I'd like to take a slice of the spectrum (say, a bit of
yellow), and compare the intensity levels directly from the sun and
reflected off the moon as part of the lab.  I suppose I could do this
directly on the white light version, but wouldn't that be more prone to
burning out the meters?

> > hence the need for a long prism.
>
> I don't see the connection.  I don't see any necessary connection between
> the size of the prism and the size of the image.

And now it's the test of ASCII-sketches.

   ----------------------------------------------------------
   {     }
   {Prism}   "Direct" path to show target image for aiming at the sun
   {     }
   ----------------------------------------------------------
   {     }                                                  :
   {     }                                                  : Grating
   {     }            Refracted Spectrum                    :
   {Long }                                              -----
   {     }                                         -----
   {Prism}                                    -----
   {     }                               -----
   {     }                          -----
   {     }                     -----
   {     }                -----
   {     }           -----
   {     }      -----
   {     } -----


To keep everything compact, I'd put the sheet of paper for the students to
mark a few inches below the long prism on the left (at a level underneath
the ASCII-rendition).  That way, I can mount the whole thing on a camera
tripod, with a clipboard or somesuch for the students to support their sheet
of paper, and off they go.

You have to have a long prism (or mirror) to put the "bend" near the end of
the path.  There's probably no "good" reason for a prism over a mirror, I
just like prisms.

Now, there will be a bit of correction need for the scaling as I understand
it, since we have different lengths due to the positioning of the prism, but
that shouldn't be too bad to figure out.

I may still need some focusing optics should the light be too dim, but I
don't think that'll be too bad a problem.


---------------
Gordon P. Smith
Visiting Assistant Professor of Physics and Astronomy
Hampden-Sydney College
Hampden-Sydney, VA 23943
(804) 223-6177