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Jim Hickey wrote:
"Quantum mechanics and celestial nav, what a mix!"

For an explication without recourse to quantum mechanics:
http://alienryderflex.com/polarizer/
that finishes: "So there you have it!  No spooky quantum properties.
Nothing very mysterious about it, in fact.  A straightforward chain of cause
and effect, yielding rational, comprehensible results."

Another site that deals with this in terms of the Law of Malus is
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polcross.html
that states that: "If the center polarizer is placed at 45? between crossed
polarizers, 25% of the light will be transmitted."

Googling with "third polarizer" turns up lots of sites. One, from a high
school in Maine, even uses the poor old picket fence analogy, going on to
state:
"The middle one re-steers the polarized wave to have vibrations
somewhat in the direction that the last one will pass."
http://www.iit.edu/~smile/ph96m6.html

This idea seems popular, presumably because it is understandable (kind of!)
even if Jim Hickey's site specifically warns against such imaginings.
Eg:
"then ... take a third and insert it between the two at say, 45o ... more
light will come through than with the two anti-parallel polarizers. This
would logically require that the third polarizer "rotate" the angle of
polariztion of some of the light which has passed the first polarizer ...
thus allowing it to go through the third." from
http://www.ebtx.com/ntx/polaroid.htm

   

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