After light passes through the first polaroid, all the waves are vibrating in
one direction (e.g., up-down, or left-right, or some particular angle). (This
is much easier to show with diagrams!!). When a second polaroid is added so
that it blocks all the light, the two polaroids are "crossed," with their
axes at right angles to each other so that the second polaroid blocks all the
waves vibrating in the direction allowed by the first polaroid. When a third
polaroid is placed between the two crossed polaroids, then you have to
consider the direction of vibration of the waves that pass through the first
polaroid as being broken into two vector components at right angles to each
other, one oriented parallel to the axes of the middle polaroid so that it
passes through, and the other oriented perpendicular to the axes of the
middle polaroid so that it is blocked. The component which passes through the
middle polaroid is then itself considered to be broken into two vector
components at right angles, one passing through the third polaroid (this is
the important one from your standpoint), and the other being blocked. Again,
this would be a lot easier with drawings! Hope it helps and is still timely.
Apologies for the delay in replying due to delays in forwarding plus the
holidays.
Don Rathjen
Exploratorium Teacher Institute
donr@exploratorium.edu
or
donrath@aol.com
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