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Re: glass ...



On Wed, 28 May 1997, Jim Pengra wrote:

One measures the angle A of the prism, and delta the angle of minimal
deviation for a selected wavelength, and then the refractive index
relative to air ( mu) is given by

mu = sin 1/2 (A + delta)/sin 1/2 A

The trouble with this method is that the minimum is practically flat over
the broad range of angles. Thus a very large uncertainty will always be
associated with delta. I nice mathematical relation, but not very useful
for measuring mu.

But it's not delta that's imprecise, it's the position of minimum
deviation. This means that you don't have to sit exactly on the position of
minimum deviation to get a good value for delta and hence a good value for
mu. The precision of delta (and A) depends only on your skill and the
quality of the angle measuring apparatus.


--Jim(pengra@whitman.edu) Walla Walla, Washinton 99362


I agree with Jim. The *reason* for using the quoted formula and for
obtaining minimum deviation is that

(1) You don't have to measure the incident angle on the first face, which
would introduce another source of experimental uncertainty.

(2) You don't have to use a *very* messy and *long* formula. (I ask
students to derive the formula for deviation angle for any incident angle.
This makes them better appreciate the cleverness behind the standard
method of use of the spectroscope, and gives them valuable practice in
using geometry.)

(3) The "flatness" of the curve of deviation angle vs. incident angle
ensures that any likely uncertainty in setting the prism at minimum
deviation will have negligible effect on the result.

I wonder what the person above expected to use instead of the formula he
quoted as "not very useful"? Does he have a better method, with less
uncertainty in the result? The world of experimental spectroscopy is dying
to hear of it!

-- Donald

......................................................................
Dr. Donald E. Simanek Office: 717-893-2079
Prof. of Physics Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745 CIS: 73147,2166
Home page: http://www.lhup.edu/~dsimanek FAX: 717-893-2047
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