Testing Polarizers
You can also easily tell if a polarizer is linear or circular even if it
is not marked.
Hold the polarizer about 2 or 3 inches in front of your eye, and look at
your reflection
in a mirror. Looking from the camera's viewpoint (i.e. with the filter
threads pointing
towards you) you should see the image of your eye in the mirror (as if
looking through a
neutral density filter). Now turn the filter around so that the filter
threads point at
the mirror. If the polarizer is linear you should see the same thing
that you saw before,
but if it is circular it will appear black and you will not see the
reflected image of
your eye. This is because light reflected from your eye passing through
the circular
polarizer exits in the direction of the mirror as circularly polarized
light (let's say
it's left handed circular polarization). When it reflects from the
mirror it reverses its
polarization to the opposite sense - in this case it is reflected as
right handed circular
polarization. The filter only passes left handed circular polarization
when the light
enters from the "camera side" of the filter, so the reflected light is
blocked.
The situation is analogous to the effect of a vertically polarized
linear filter on
horizontally polarized light - little or no transmission of the light
can occur.