miaim@xxxxxxxxxxxxxx wrote:
> Regarding comparison of circular and linear polarizers, I note that both my
> Tiffen marked "circular polarizer" and my unmarked Vivitar (presumably
> linear polarizer) have what appears to be a single piece of glass. At least
> a bit of lint applied to the camera side rotates with the front side and
> the apparent thicknesses would seem to indicate only one piece of glass. Is
> the Tiffen marked wrong or do they use another process?
All the glass in the circular polarizer should rotate together.
The circularizing element is actually a special anisotropic
crystal. When light passes through this crystal, its speed
depends on its direction of polarization relative to an axis in
the crystal. In order to convert the linearly polarized light
from the polarizer into circularly polarized light, the polarizer
must be aligned properly with the axis of the crystal. The
alignment should be at 45 degrees from the axis:
crystal
axis
| / polarizer axis
| /
|/
+---- 90-degrees from crystal axis
The component of the light along the crystal axis moves faster
than the component perpendicular to that axis. As a result, the
phase between these components shifts as the light travels
through the crystal. By making it the proper thickness, one
can get a 1/4 wavelength shift -- such a crystal is called
a "quarter wave plate" for this reason.
Anyway, circularly polarized light is light with polarizations
in two perpendicular directions, one 1/4 wavelength ahead of
the other in phase (or 90 degrees ahead in phase). That's
exactly what the quarter wave plate does to linearly polarized
light.
If the linear polarizer is not aligned properly with the
1/4-wave plate, then this doesn't work. If it's parallel
or perpendicular to the crystal axis, you just get linearly
polarized light out. If it's not 45-degrees between them,
then the amplitudes of the out-of-phase components will
not be equal and you get elliptical polariazation.
So you probably can't tell by looking at the glass whether
your polarizer is linear or circular. If you have two identical
polarizers, you can tell by looking through them both. If
they are linear polarizers, you will be able to see through
them when the axes are aligned, and they will become opaque
when the axes are at right angles. With circular polarizers,
the relative orientation won't matter -- you will see no
change as they are rotated relative to each other.
joey
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