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Re: TIR in periscopes



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