Chris Barrett wrote:
> Now, a linearly polarised photon is actually a mixture of two circularly
> polarised photons (left and right handed). The electric (or magnetic) field
> vector is not oscillating in a plane as for a linear photon, but is rotating
> either clockwise or anticlockwise.
>
> The circular polariser selects one 'handedness' type of photon rather than
> the other.
As long as we're being pedantic . . . I don't think this is right either.
A circular polarizer consists of a linear polarizer followed by a
quarter-wave plate. The linear polarizer selects exactly the same
components of the incoming light that any linear polarizer does (i.e.
the effect on the scene you are photographing is the same). The quarter
wave plate then splits the linearly polarized light into two components
(each at 45 degrees to the axis of polarization), and retards one
component by a quarter wavelength with respect to the other. This has
the effect of turning the linearly polarized light into circularly
polarized light. So the selection criterion for the filter is the
linear polarization state, not the circular one.
This distinction is important (if you happen to care about how these
things work). A circular polarizer that actually did select one
handedness of incoming photon would always have the same effect on
linearly polarized light (always passing half of the light),
regardless of the orientation of the polarizer, while the linear
polarizer + quarter-wave plate will pass anywhere from 0% to 100%,
depending on orientation (for a perfect polarizer and perfectly polarized
light, of course). The latter is the behavior you want for
photographic purposes.
Steve Schaffner
sschaff@xxxxxxxxxxxxxxxxx
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