Haidinger's brush

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. Many animals, mostly invertebrates, can sense polarization⁴. Humans can too, through an accident of anatomy in the eye¹.

The macula, the central patch of the retina, is covered by a layer of yellow pigment. The pigment molecules are laid out in rings around the centre of the macula¹. 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 in some directions around the centre and less in 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 brush⁴. The average threshold was 56% polarization⁴. Some people saw it down to 23%⁴. On average, people did better than any other vertebrate tested so far⁴.

How common it is

Most people can learn to see the brush, but few notice it without being told what to look for and how¹⁴. A 2015 study measured how faint the polarization could be before people lost the effect⁴. Its 27 volunteers were students and colleagues, not a random sample of the public⁴. Each had a short training step first, and 4 of them did not pass it⁴. So “most people, with practice” describes a small group in a lab after short training, and no broad population survey of first-time success exists.

Try it

1. Open a blank white page on an LCD monitor, tablet, or phone. Many phone screens are polarized. If one is not, try another.
2. Look at the centre of the screen. Relax.
3. Slowly tilt your head to one side, then the other.
4. A faint yellow bow-tie, or a faint blue one, appears at the point you are looking at. It rotates as you tilt.
5. 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.

Is it a problem?

No. Seeing Haidinger’s brush is a normal quirk of retinal anatomy, not a sign of eye disease¹. Not seeing it, even after trying, is also normal, since the effect is faint and takes practice to notice. Eye clinics sometimes use the brush in training. It helps some people with strabismus (a squint) learn to look with the centre of the retina¹.

History and culture

The effect was described in 1844 by Wilhelm Karl von Haidinger, an Austrian scientist¹. One hypothesis says Viking sailors found the sun on cloudy days using crystals called sunstones. Some versions of the hypothesis have sailors use the crystal together with Haidinger’s brush². The hypothesis remains unproven². The XKCD “Visual Field” map includes the brush and says it appears at the centre of vision³.