Ask A Scientist Physics Archive

name         Jess V.
status       educator
age          50s

Question -   Why does the "axis" of a Polaroid lens appear to
shift by a quarter turn (90 degrees) for light traveling in opposite
directions? This behavior does not seem to fit the standard "picket
fence" model for polarized light.
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This is not the behavior of a typical polarizing film.  Please make sure
you are doing the experiment correctly.

It requires two pieces of polarizing film and a light source such as a
flashlight.
     you must know the orientation of at least one film's polarization axis

Put the two pieces of polarizing film in front of the light

Rotate the polarizing film closest to the light so that most of the
light is transmitted through both pieces of film.

Turn the other polarizing film around so that light is passing through it
in the opposite direction.  It is very important that you not change the
orientation of the polarization axis of the film when you do this.  If
you happen to rotate it about an axis that is at 45° to the true
polarization axis of the film it will appear as though the light
transmitted has changed orientations.  If this happens, try turning it
around about an axis 45° away from the one you originally used.  This
will either be the polarization axis or at 90° to the polarization axis.

There is no guarantee that the polarization axis is aligned with an edge
(or geometrical axis) of the piece of polarizing film.  This will depend
on how it was cut from the original sheet.

Greg Bradburn
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