Message-Id: <v01530500af5cdc854742@[24.1.66.176]>
Date: Mon, 24 Mar 1997 18:07:37 -0800
To: pinhole@exploratorium.edu
From: pgeschke@inow.com (Peter Geschke)
Subject: Polarizing filters
Hopefully, this one is not too superfluous:
Recently my class and I were discussing polarizing filters. They each had
a set of two cheapies to play with and I had the students take a third
filter and put it "diagonally" between two filters so that the "triangle"
formed will either allow some light through filters that are 90=B0 to each
other or block some light through filters that are "in line".
My explanation for this was not well received, but I believe it to be
correct. Perhaps someone has another way of explaining this that can help
me/them understanding this phenomenon better.
My way of explanation is to say that when you take the first "right angled"
filter (A) and place it next to the "diagonal filter" (B) some light is
getting through. The source of light we are using is nonpolarized, such
that electrons are vibrating in all planes generating light waves that are
traveling in all planes. We simplify this monstrosity by saying that
nonpolarized light travels in just two planes: vertical and horizontal, and
after passing through a polarizing filter, light travels in just one plane,
say vertical.
By rotating filter B 45=B0, we are allowing all of the vertical light throug=
h
the filter A, but only "1/2" of that vertical light through the filter B -
thus some of the light does not get through.
By placing another "right angled" filter (C) that allows horizontal light
through behind filter B we still allow some of the horizontal light through
filter B. Just to be clear (I hope), if filter B was not between A and C,
then no light would be transferred "all the way" through A and C.
Since some light gets through A and B, and some light gets through B and C
- therefore, some light gets through A and C when all three are lined up as
described. This also works if filters A and C are both allowing vertical
light except that B serves to block some light instead of allowing all
light through.
This last paragraph seems to be the part of the explanation that they are
getting lost on - what am I missing or how can I improve my explanation?
Thank you very much for any input you can provide,
Peter Geschke
Mission San Jose High School
=46remont, CA
510-657-3600 X3528
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