Haidinger's brush
Also called: Haidinger's brushes · Polarization brush
Humans can see the polarization of light without any tool. It appears as a faint yellow bow-tie in the middle of your vision when you look at a white screen or a blue sky.
How common
Does this happen to you?
These are answers from visitors, not a study.
How it works
Light is a wave. Polarized light is light whose waves all vibrate in the same plane. Light from a blue sky and from most LCD screens is strongly polarized. Most animals that see polarization have special eye structures for it. Humans do not, but the human eye has an accident of anatomy that does the same job1.
The macula, the central patch of the retina, is covered by a layer of yellow pigment. The pigment molecules are laid out in a radial pattern, like spokes on a wheel. Each molecule absorbs blue light most strongly when the light’s plane of vibration lines up with it1. So polarized blue light is absorbed more along some spokes and less along others. The result is a faint pattern: a yellowish bow-tie about 3 to 5 degrees wide, with a bluer bow-tie crossing it at right angles1.
The brush is very faint and it fades in seconds because the retina adapts to it. To keep seeing it you have to rotate the light source or tilt your head1.
A 2015 study measured how weakly polarized the light can be before people stop seeing the brush1. The average threshold was 56% polarization1. Some people did better than any vertebrate previously tested1.
Try it
- Open a blank white page on an LCD monitor, tablet, or phone. Many phone screens are polarized. If one is not, try another.
- Look at the centre of the screen. Relax.
- Slowly tilt your head to one side, then the other.
- A faint yellow bow-tie, or a faint blue one, appears at the point you are looking at. It rotates as you tilt.
- If you see nothing, try a clear blue sky at right angles to the sun. Never look near the sun.
Once you have seen it, it gets much easier to find.
History and culture
The effect was described in 1844 by Wilhelm Karl von Haidinger, an Austrian mineralogist1. One hypothesis says Viking sailors found the sun on cloudy days using crystals called sunstones. That hypothesis is about the crystals, not the brush, and it remains unproven2. The brush appears on the XKCD “Visual Field” map at the exact centre of vision3.