Larry asked: Isn't the silvering behind the glass?
Brian replied that the mirror is a "front" (first) surface mirror
because a second surface mirror would give two reflections. One, a weak
reflection from the glass surface and the second from the silvered
surface at the back.
Another point is that, if the light were to pass through the glass
before being reflected back through it again there would also be some
degree of refraction. Because of refraction and double reflection
optical instruments of any type that use mirrors use a first surface
mirror. The surface is also typically coated with aluminum rather than
silver. Silver is more reflective than aluminum but oxidizes quickly
and the oxide is opaque (silver tarnish). Aluminum also oxidizes very
quickly but aluminum oxide is transparent and hard (used in grinding
wheels). The aluminum oxide forms a hard, protective coating over the
mirror but it is microscopically thin.
Silver used to be used on telescope mirrors many years ago but it always
required some sort of protective coating. The coatings were far from
perfect and the mirrors would have to be periodically stripped and
resilvered. Ordinary household mirrors are second surface mirrors. If
silver is used it is protected from oxidation on one side by the glass
and on the back side by a heavy layer of paint.
There are some oddball telescope designs where a second surface mirror
is used on the backside of a lens. Here, the light enters the lens and
is reflected right back out through the same lens. There is certainly
refraction involved but it is refraction by design. I'm not familiar
with the optical design of the Vivitar "solid cat" (catadioptric lens)
but from the name I would infer that it may be one of these second
surface mirror designs.
Chuck Norcutt
Woburn, Massachusetts, USA
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