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Re: TIR in periscopes

Michael's response to the question about the use of prisms instead of
mirrors in a periscope is one of the best responses that I have ever
read. It gives a thorough coverage of the subject using simple language,
clear writing and good grammar. He uses numbered paragraphs, each
starting with a sentence that captures interest followed by sentences
that develop the theme with simple explanations and clear reasoning. It
is unusual to find such a beautifully written e-mail these days when
everyone is in such a hurry to get on with more pressing issues that come
up each day.
Wouldn't it be nice if a whole physics textbook could be written with the
same approach?

Herb Gottlieb from New York City
(Where clear thorough explanations are appreciated)

On Wed, 16 Feb 2000 09:24:41 -0500 Michael Edmiston
<edmiston@BLUFFTON.EDU> writes:
Here are a few reasons for using prisms instead of mirrors in a
periscope.

(1) In order to avoid double reflections from a mirror, the mirror
has to be
a front-surface mirror. Although front-surface mirrors are used in
many
optical systems, they are more susceptible to corrosion than prisms,
and
they are more difficult to clean (because they're easy to scratch).
The
point here is that a prism is much more rugged than a front-surface
mirror.
Although some might argue that points made below are more important,
I think
ruggedness is the number one criterion, so I list it first.

(2) It is easier to align the prism (and keep it that way) than to
align a
mirror. If we want a 45-degree reflecting surface the prism will be
ground
to a 45-90-45 triangle and this will remain fixed. Then the prism's
flat
entrance and exit surfaces make nice reference planes for alignment
and
mounting. I would say this is the second most important criterion.

(3) The total internal reflection in a prism yields a higher
percentage of
light reflected than a silvered surface which will absorb some of
the light.
Note that a prism is not guaranteed to be better here, because we
have to
worry about reflections from the entrance and exit of the prism.
But these
prism surfaces will be coated with antireflection coatings to cut
down on
that.

(4) This is somewhat a continuation of the last point. Depending
upon the
prism's glass versus the mirror's coating, the wavelengths of light
being
used could make a mirror better than prism for some wavelengths.
For
visible light it is pretty clear that a prism gives a flatter
transmission
curve across the spectrum.


By the way, perhaps a more common example than periscopes is the use
of
prisms in binoculars. Two 90-degree bends at right angles to each
other can
turn the normally inverted image of a "telescope" into an erect
image. The
typical set of binoculars has two 45-90-45 prisms in each side, four
prisms
total. This causes the obvious offset in many binocular tubes.
Some
binoculars which still provide erect images, but do not have offset
tubes,
have more complicated prism systems to provide the erect image.
This makes
them more compact, but also more expensive (when comparing two types
yielding similar quality images.)


Michael D. Edmiston, Ph.D. Phone/voice-mail:
419-358-3270
Professor of Chemistry & Physics FAX:
419-358-3323
Chairman, Science Department E-Mail
edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817