Recently I have been cultivating my ability to see Haidinger's
brushes. This is a peculiar visual effect that gives some people
the ability to detect the polarization of light with the unaided
eye!
It looks like a faint yellowish bowtie, or, more accurately, a
yellowish bowtie and a bluish bowtie at right angles to one
another. The blue bowtie can be a little harder to see, and many
drawings depict it as a pair of faint blue spots. The colors in my
drawing are greatly exaggerated. If you stare at my drawing for
twenty seconds or so and then avert your eyes, the afterimage
you'll see is pretty close to the way it really looks (except that
it doesn't behave quite like the real thing: read on).
The axis of the blue bowtie is in the plane of the electric
field of the polarized light waves (what we conventionally call the
polarization plane), and the axis of the yellow bowtie is in the
plane of the magnetic field. If you look at a source of polarized
light, the bowtie-thing floats in the middle of your visual field
and stays fixed with respect to your eyes-- but its
orientation stays fixed with respect to the outside world
if you rotate your head. It is an odd and eerie effect.
Some people can see the effect and some people can't. I suspect
that most people actually can, but haven't tried. Its origin turns
out to be a bit mysterious; it's unclear precisely how it works,
but there appears to be material in the human eye that transmits
different polarizations differently. This long review article on
polarization and the eye (PDF; requires Acrobat Reader or
equivalent) talks about what is known.
Here's
a very short page on Haidinger's brushes, with a big
drawing.
This
longer page has some historical information.
I had read about this phenomenon before, but never actually seen
it until one day when I saw it quite by accident.
Most descriptions of Haidinger's brushes you'll find in books or
science course materials are some years old, and tell you to look
through a sheet of polarizing film-- which is in plentiful supply
in school physics labs, but is not so common around the house,
especially since polarizing sunglasses aren't so popular these
days.
But there is another, increasingly common source of polarized
light. A typical laptop computer screen (or one of the flat-panel
desktop screens that are all the rage lately) is a nematic liquid
crystal sandwiched between crossed sheets of polarizer. The liquid
crystal transmits or blocks light by twisting, or failing to twist,
the polarization plane of the light. All the light it emits is
highly polarized-- that's how it works!
A co-worker of mine was showing off a new Apple iBook when I
noticed that I saw a strange bowtie-shaped color distortion on the
screen-- one that only I could apparently see. It took a minute for
me to realize that I had finally seen Haidinger's brushes.
The only other page I've found so far that mentions seeing this
on laptop screens is this Japanese language page on
light polarization, but there could well be others-- lots of
people spend time looking at these LCDs!
Rotate your head!
Display a fairly solid, white or light blue image on an LCD
screen (a CRT monitor will not work), then stare at the image and
tilt your head (or the screen) slowly. You may be able to
see the color bowtie pattern. The motion makes it easier to see
Haidinger's brushes because the image will appear to rotate
relative to your eyes, so your eyes and brain will be less likely
to edit it out, as they will an image that remains unchanged with
time.
The light from an iBook screen turns out to be diagonally
polarized. I'm not sure whether this is universal, but it seems to
be a common thing.
The light from the blue sky is somewhat
polarized, in a plane perpendicular to the direction to the sun. I
had heard that Haidinger's brushes can be seen faintly in the sky,
but I never managed to see them, even after the laptop incident,
until a few days ago.
My wife, Samantha, plays French horn in the excellent Middlesex Concert
Band. Once every summer, the band plays a free pops concert at
the famous Hatch Shell on the
Charles River Esplanade in Boston. In 2001, Sam's sister, Lindsey,
had the foresight to bring a blanket so we didn't have to sit on
the ground. The concert happened near sunset, when I had heard that
the seeing of Haidinger's brushes is best. So at one point I lay
back and tried to see the brushes in the sky.
Sure enough, after a while I spotted the effect. I found that
just as with the laptop, it helps to rotate your head. It's a much
fainter effect than with the laptop because the light is more
weakly polarized; it doesn't jump out at me immediately every time
I look. But it's there, and is most clearly visible at ninety
degrees to the sun's position. The yellow bowtie points in the
direction to the sun, since it's perpendicular to the polarization
plane.