While on a long, boring drive on I-5 I noticed that the scenery was easier
to visually interpret if I tilted my head to the left, and harder if I
tilted right. As I was tilting my head back and forth, I noticed
something I'd never seen before. The blue sky out towards the horizon was
distinctly changing in brightness, yet the hills and trees were not.
The blue sky on a clear day contains partially polarized light. If you go
outside with a sheet of polaroid plastic, you can rotate it while looking
at the blue sky, and a particular zone in the sky will become dim and
bright. This zone forms a huge band which arches across the sky at 90
degrees from the location of the sun. E.g. at sundown this band of
polarization sweeps from the western horizon, up to the zenith, then down
to the eastern horizon. The sky in this band is fairly dark blue, while
the sky towards the sun and opposite the sun is much lighter and
unpolarized.
Well, I find that as I tilt my head, the location of the entire band seems
to shift. On a sunny day in the early morning or late afternoon, look
east or west, then tilt your head back and forth. Pay attention to the
darkness of the blue sky. Does the darkest part move along?
This suggests to me that my retina is somewhat polarization-sensitive. At
least one of the four color-sensors is.
There is another polaroid/retina phenomena too, called Heidengers(sp?)
brushes. If you look at a broad polarized light source having a rotating
angle of polarization, you will see a small yellow/blue shape in the exact
center of your visual field. It's about the same angular size as the sun,
and is very fuzzy (hard to notice unless you look for it.) I'd seen this
mentioned in an old C.L. Stong column, but I never heard that the whole
retina was polarization-sensitive. Or is it just me, and most people
DON'T see the sky change when they wave their head back and forth.
Also: if kids point at you and laugh when you stand out in public waving
your head while staring into space, here are some more visual demos which
will accomplish even more embarassment: