WARNING: PHYSICS POST
Maybe I should have “Physics Fridays” or something.
Haidinger’s brush might be my favorite part when I teach about the polarization of light. (If you have no idea what that is, click the polarization link or find a Physics textbook.) Most people don’t realize that you, with just your naked eye, can tell not only if you’re looking at polarized light, but also how (horizontally, vertically) the light is polarized.
If you’re looking at polarized light, you should see what looks like a yellow bowtie crossed with a blue bowtie at the very center of your vision (see graphic, courtesy Wikimedia Commons). Personally, I can only really see the yellow. The light you’re observing is polarized perpendicular to the yellow bowtie (or parallel to the blue).
In order to see Haidinger’s brush you need to be looking at polarized light – I have students look through a polaroid filter first. They then rotate the filter, which also makes the bowtie rotate – that makes it easier to see. Once you learn to see it, try it without the filter when looking at another source of polarized light – say, the LCD monitor you’re staring at right now. If you rotate the monitor (careful now) the bowtie will rotate too!