I like to demonstrate Haidinger's brushes to my students, especially
those who think that physics is of no importance in biological science.
I use a fairly large polaroid sheet, though once one knows what to look
for the effect may be easily seen with much less specialized equipment.
I occasionally see it when I'm not even looking for it.
The clear sky 90 degrees from the apparent position of the Sun is
partially polarized. If one looks through a polaroid sheet at this sky
and rotates the sheet quickly back and forth obout the direction of the
gaze, the brushes (which I see as a small pale purple bow tie pattern
on a pale yellow background) will be seen to be rotating with the
sheet at the center of the field of view. The same effect can be seen
by walking around in a circle while holding a mirror, looking at the
zenith in reflection with the Sun low in the sky.
The eye is an AC detector, which is why one does not see the brushes
all the time. The rotation of the polaroid is necessary to provide the
AC. (This also accounts for the usual invisibility of floaters.) Bees
are known to use the polarization of skylight for navigation purposes.
Stress patterns of birefringent automobile windows (especially rear
windows) can often be seen fortuitously when observed in polarized
skylight illumination. Reflection from the glass usually serves as an
analyzer in these cases. Of course the phenomenon is seen much more
frequently by the polarized sunglasses wearer.
Bill Beaty is a keen observer of many phenomena that occur in the
mundane environment most of our students take for granted. Teaching our
nonmajor students to observe - to become attuned to their environment -
is, perhaps, our most important function as physics teachers. Teachers
of "natural history" biology recognize the great importance of this
function, and even physicists take some interest in being able to tell
the differences between different species of wildlife. Somehow physics
teachers frequently miss the point that interesting physical phenomena
are at least as common in the environment as interesting biological
phenomena. My own inclination is to interrogate my environment
occasionally. In particular, I like to tap or even hit various
structures to find out how they ring. This may seem like odd behaviour,
but who cares? It is interesting, and that's all that matters.