Haidinger's Brush

In 1846, the Austrian physicist Haidinger reported the subtle visual impression produced by polarized light on the eye. Brewster and Stokes studied the phenomenon and commented on it, but it has remained one of the most obscure visual observations. It was described by Minnaert in Light and Colour in the Open Air, pp. 254-257, and if you can possibly read it there, you should, if only to become acquainted with a wonderful book.

When polarized light enters your eye, you briefly see a small spot near the center of the visual field that looks to many observers like a yellowish double-ended brush with blue clouds on either side. The yellowish brush is perpendicular to the electric vector of the polarized light. The figure is stronger as the light is more completely polarized. Unfortunately, like all small disturbances of the visual field that the brain knows is not really there, the spot soon disappears unless an effort is made to regenerate it by averting the vision. The spot cannot be seen by staring at it! All observers find that it requires much practice before the brush can be reliably seen. Minnaert could do it well.

Sensitivity to polarization is an effect of refraction and reflection, or to dichroic molecules that absorb differently depending on the direction of polarization relative to a molecular axis. In the case of Haidinger's brush, dichroism of some point in the yellow spot, or macula, of the retina is blamed. I have not heard of any recent research on this topic that would shed a brighter light on the source of Haidinger's brush.

Minnaert says that the brush is more easily seen in a garden globe, because the contrasts are made more vivid. Others report that it is more easily seen through a green or blue glass, but disappears in a red or yellow glass. Different observers are reported to see the spot differently, which is not surprising, since its source is in the eye. Sometimes the yellow is continuous, and sometimes the blue. Some say the continuous color is the one most perpendicular to the line joining the eyes. In sky light, the yellow brush points in the direction of the sun. This property has been used to locate the point on the horizon where the rising sun will first appear, to give a better chance of observing the green flash.

Blue skylight is polarized because it is light scattered from molecular fluctuations in the upper atmosphere, and the polarization is strongest at 90° from the sun. Light from clouds, or from a hazy sky, is not polarized, and neither is the violet glow at twilight. The rainbow is polarized, but its light is not suitable for seeing Haidinger's brush. Light reflected from a dielectric plane (glass, water, mirage) is polarized with its electric vector parallel to the reflecting surface. At the polarizing angle, the reflected light is highly polarized. For glass, this is a glancing angle of 34°, for water, 37°. Polarized sunglasses absorb light polarized horizontally to reduce glare. Therefore, one can practice looking for Haidinger's brush in the light reflected from a plate of glass. When you rotate the glass, the brush should rotate as well. A Nicol prism is excellent for the purpose, but they are now very hard to find. A common polaroid can be tried, but perhaps the color will interfere.

If you want to see Haidinger's brush, here is a procedure that might work. Find a linear polarizer that is as color-free as possible. A Nicol prism is best, but almost unobtainable these days. A good-quality dichroic filter will also work. It should look gray, not green, blue or orange. Look at a piece of white paper under rather strong white light. Close one eye and put the polarizer before the other. Now rotate the polarizer back and forth, and look for a delicate small pattern in the center of the field of vision that rotates with the polarizer. It will look like an hourglass shape of yellow with blue spots on each side. With some eyes, the colors may be reversed. Whatever they are, if it rotates with the polarizer, it is Haidinger's Brush. Remember that concentrating on the pattern makes it disappear.

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Composed by J. B. Calvert
Created 1 July 1999
Last revised 11 January 2001