Hi.

I only found out about this ("Haidinger's Brush") on the net yesterday,
although I had previously seen cryptic mentions in various places that
some people are able to detect polarisation of light with their unaided
eyes, especially in the blue sky of the evening. Something about some
people being able to see "crosses" in the sky. "Huh?" I said to myself,
"I've never seen 'crosses' in the blue sky!" Occasionally I tried looking,
but never saw anything.

Anyway, yesterday I saw this name "Haidinger's Brush" mentioned, so
googled for it and found various similar pages and explanations,
including this one:

http://www.polarization.com/haidinger/haidinger.html

(So we're not looking for "crosses" anyway!)

So last night I tried it with a polarising filter and my (CRT) monitor
displaying flat white. Sure enough, it wasn't long before I could see the
(mainly dirty-yellow) "brush" shape and some evidence of the blue "brush"
perpendicular to it, as I rotated the polarising filter. I was pretty
surprised and satisfied!

Apparently, and this makes sense, though I haven't been able to try it yet,
LCD colour monitors (of course) emit polarised light, and the "brush" is
visible on white backgrounds without any extra polarising filter.

I just tried it with my mobile phone (Ericsson T610) which has a colour
LCD display, displaying a jpeg of nothing but flat white. Sure enough,
there's the brush! Never would've noticed it before! The polarisation is at
something like 45 degrees, with the yellow "brush" running from top-right
to bottom-left, if I hold the phone in its intended orientation. If I rotate
the phone, the "brush" rotates with it, and does not stay fixed going from
my view of "top-right to bottom-left".

Now I want to try it on blue sky. Today was mainly overcast with a few blue
gaps, so I couldn't convince myself I'd seen it.

Really, I posted this because I know many of you learned optics gurus will be
well familiar with this effect, and perhaps may have some other interesting
tales to relate.

One final observation, and I don't know whether this is just my imagination,
or some unrelated effect: Even when I look at a bright UNPOLARISED flat
white image, I seem to see some dirty yellow staining in a small spot around
the fovea. I wonder if this is related to the dirty yellow brush, but rotated
round so that there is no preferred direction, because of the lack of
polarisation. Or maybe it's just that the white is bright and affecting the
sensitivity of the foveal area, making that part appear less bright than it
does at areas away from the fovea.


Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.fleetie.demon.co.uk