I should have mentioned that circulars are almost always marked as such with
"Circular" or "C-POL" or something else other than simply "Polarizer" or "POL."
What occurs in the mirror test through a circular:
In the proper direction, with the front (polarizer) facing your eye and the
back (quareter wave plate) facing the mirror, the light path is affected
thusly, and makes the polarizer appear as a completely opaque black disc in
the mirror:
(a) Light reflected from your face around your eye and traveling toward the
the mirror is first linearly polarized by the polarizer.
(b) It's then circularly polarized by the quarter wave plate. Circular
polarization also has an orientation as either "right-hand" or "left-hand."
(c) The circularly polarized light is then reflected by the mirror, and a
mirror *reverses* the orientation of circularly polarized light. If it's
RH when it strikes the mirror, it's LH when it's reflected, and vice versa.
(d) The light then attempts to enter the quarterwave plate and polarizer
again on the way back to your eye. Since it's now the opposite circular
polarization it is blocked by the quarter-wave plate, doesn't pass through,
and the polarizer appears as if it's a pure black disc.
Pure linear polarizers don't do this. It's also why the circular test only
works in one direction. If you look through a circular with the front
(linear polarizer side) facing the mirror you can see your eyeball through
it. The light striking the mirror is linearly polarized because the linear
side is facing the mirror. The mirror reflection returns it linearly
polarized, and on the return path it passes back through the linear side
facing the mirror before it hits the quarter wave plate.
-- John
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