Re: Glass....

[Leigh Palmer <palmer@sfu.ca>]

> > > I have a related question.  Which glass are the prisms we buy from Edmund,
> > > Cenco, Pasco, Fisher, etc.?  The catalogs don't say, nor does any
> > > documentation I can find.  I'm looking for the index of refraction of the
> > > prisms in a spectroscope experiment.
> >
> > Don't you measure the index in the experiment?
>
> Yes, and the value came out between the listed values for crown and flint.

I'm not too surprised, but prisms sold for that purpose are usually dense
flint glass because it exhibits high dispersion. The index of refraction
is what you measure it to be, including the uncertainty in the measurement.
When I last taught that lab I believe I was getting three places of
decimals by the minimum deviation method. That was using an accurate
goniometer with verniers. The position of minimum deviation is not so
uncertain as Ludwik makes out. You should use magnification to measure it.
Getting it crudely using protactor and a laser will, of course, yield
correspondingly crude results.

If you do use a laser then you should try to exploit the magnification
inherent in long path length. With the laser beam parallel to and a few
centimeters from a side wall, mark the beam center on an end wall*.
Now place your prism close to the laser and rotate it until you reach
the angle at which the spot on the end wall is nearest the first point.
This is a very sensitive method, so rotate carefully. Mark the beam
spot again. Measure D, the distance from the end wall to the *center*
of your prism. Ideally you should place the prism initially with both
incident and refracted beams projecting through this center. Measure
the distance d between the two marks on the wall. The angle of minimum
deviation is the arctangent of d/D. The apex angle of the prism can be
accurately measured by directing the laser beam at the edge and making
two reflected spots on the wall and measuring these, but you will
likely find that the apex angle is 60 degrees to better than your
uncertainty. It is worth a check, however, just because it is so easy
to do.

Leigh

*What you really want is to be perpendicular to the end wall. If the
room is accurately square then making a horizontal laser beam parallel
to the side wall will accomplish this. I have recently been involved
in the frustrating task of mapping my garden without a transit. It is
clear that humans have a strong disinclination to make square rooms.