Haidinger's brush

In 1844 Wilhelm Ritter von Haidinger noted in the annals of physics on a hitherto unobserved ability of the human eye: With the perception of an inconspicuous, colored appearance - later named as a Haidinger Clump after him - most people are able to detect the polarization state of visible light to some extent. The trained observer, it is possible to distinguish between both linearly or circularly polarized light, and to identify the direction of polarization, and to evaluate different levels of polarization.

The Haidinger Tuft is an entoptic phenomenon, that is, it only is in the eye of the observer and therefore can not be photographed.

Description of the phenomenon

An observer looks for a few seconds quietly persisting in a direction that is true of the linearly polarized light to the eye. If he then - still looking in the direction - tilts his head to the side, briefly appears the so-called Haidinger - tufts. You then tilts the head alternately to the left and the right, the phenomenon can cause again.

Applies linearly polarized light having constant polarization direction onto the eye, this light is indistinguishable from natural, i.e., non-polarized light. However, changes the direction of the polarization with respect to the eye abruptly and then remains constant, the polarization tufts for a few seconds seems to fade shortly thereafter (similar to an afterimage ) again. In the event that the polarization direction rotates continuously, the Haidinger - tuft is permanently visible. It then does not fade and is rotated in the same direction as the polarization.

Observers describe the Haidinger - tufts as diffuse, yellowish form that is constricted in the middle and is cut vertically by a corresponding bluish purple shape in the middle (like a four-leaf clover ). The appearance is very unobtrusive, so it is recommended for observing a plain background without distracting pattern. The two noticeable colors ( bluish- purple and yellow ) are complementary colors. Depending on the observation situation, it may be different for the same observer, whether he sees the yellow or the bluish arm of the tuft as a continuous strip.

If the observer waits long enough until the appearance fades and then the view turns to an unpolarized source, a negative afterimage will appear shortly, with the former yellow arm now. In blue and vice versa

The orientation of the double tuft blue corresponds to the polarization direction of the incident on the eye of the linearly polarized light.

Haidinger the tufts comprises a visual angle of about 3 ° to 4 °, that is, the expected diameter of which corresponds approximately to the width of two juxtaposed fingers, which are considered in the distance of the extended arm. Maxwell gave a first indication of the cause of the phenomenon lies in the eye. It seemed to him " the area which is covered by the polarizing figure ... to size the non-vascular yard of the yellow spot right away."

In order to perceive the tuft, the degree of polarization of the incident light must be at least 60 % and - as Stokes found out - must necessarily blue, polarized light ( wavelengths less than 500 nm; contained in white light) falling on the eye. The effect appears amplified in pure blue light, yellow is then replaced by a dark blue sensation.

For right-handed circular polarization of the ( upright ) observer sees in the left and right eye the yellow tufts with its longitudinal axis from right - to left - aligned above the bottom, at an angle of about 45 °. The image is fixed with respect to the retina, and only rotates when the observer tilts his head. In the left circular polarization, the angle is about 45 °, so that the left and right figure appear mutually rotated by 90 °. The orientation can differ somewhat between the right and left eye.

Emergence of Haidinger - tuft

With the results of previous research can not be explained all aspects of the phenomenon. Physiologists suspect the cause of the phenomenon mostly in the radial arrangement of the fovea ( fovea centralis ) diverging nerve fibers in combination with the pigmentation ( xanthophylls ) in the yellow spot ( macula). This combination acts as a radially symmetric polarizing filter, that is, depending on the polarization direction of the incident light on the fovea only certain areas to their excited and result in a noticeable effect on humans.

Xanthophyll molecules have a strongly anisotropic behavior due to their rod-like structure and are thus sensitive to the polarization of light. An incident on a molecule electromagnetic wave can stimulate both electronic states and vibrational states in this molecule and indeed particularly strong when the direction of the electric field component of the light wave ( polarization direction) is oriented parallel to the longitudinal axis of the molecule.

The figure shows the arranged in concentric circles pigment molecules. Parallel to the direction of polarization of the incident light aligned xanthophyll molecules ( in the circular section B) to absorb most of the light. In Sector A there is exactly the reverse. This produces the typical form of the polarization bundle. In this case, the region B yellow and blue A ( complementary color ) appears.

The image faded then due to fatigue and the associated development of a negative afterimage (because of the nature of the entoptic effect ), which lays exactly over the original. Because the effect is weak, the two images wash each other out, unless the polarization is eliminated suddenly, after which there is only the visible afterimage. If the polarization plane suddenly turned 90 degrees or the direction of rotation ( the circular polarization ) suddenly changed, the afterimage will strengthen the refreshed image.

Application in ophthalmology

The Haidinger - tufts appear exactly in the direction in which shows the foveola and is also recognized by only this. For eccentric fixation thus the tuft is seen next to the fixation point. This property of the phenomenon makes the ophthalmology at the fixation test and pleoptischen exercise treatments for training foveolarer perception advantage.

Special ophthalmic devices called Haploskope, can make the Haidinger - tufts become more apparent by rotating polarization filter and an extra cobalt blue filter. It appears in this case as a blue vortex or " propeller ".

Observation opportunities

Many people have difficulty to recognize the Haidinger - tufts immediately on the first try. The actual quality of its appearance is much weaker than in pictures, and it has the tendency to appear and disappear again. The famous German physiologist and physicist Hermann von Helmholtz said: " I ​​have myself 12 years ago, immediately after Haidinger 's discovery with the greatest effort can not see any sign of the tufts, and lately, when I tried it again, I saw her for the first view through a Nicol prism. "

The following methods operate partly technical aids, but only serve to present the eye linearly polarized light. If you train in the beginning several times a day for a few minutes, the then already familiar figure of Haidinger - tuft is evident after one to two days without much effort.

Liquid crystal displays

LC displays emit linearly polarized light, which allows in a well darkened room a straightforward observation of the phenomenon. Is fixed to the eye a point on a white surface area on the screen (for example, an empty browser window ), the Haidinger - tuft is usually ( after a short familiarization with the delicacy of appearance) can be seen. It is recommended to start with a distance of about 50 cm, since the tuft is then easier to find on the basis of its size. The head is tilted sideways ( placed on the shoulder) to a few seconds persisting to look at the white surface. The tufts appear only when you now lays her head on the other side, while still looking at the screen. To observe is the diagonal position of the tuft (many displays are diagonally polarized to facilitate use with vertically polarized sunglasses ) and the size change when varying the distance of the eye from the monitor. In diagonal polarization of the display and the Haidinger - tufts appear diagonally in front of the eye of the beholder. The contemplation of the empty, white color LC displays, for example, a mobile phone or tablet computer is more simple because only the display itself must be rotated instead of the head and upon continued rotation of the polarization tuft remains visible.

Polarization filter

The phenomenon appears when looking through a linear polarization filter on a bright white surface (eg illuminated sheet of paper, white cloud ). Turning the filter, changing the polarization direction of the incident on the eye light continuously and so the tuft otherwise only briefly appears rotated in the same direction with and perceive light and permanently by this movement. As many sunglasses comprise vertical polarizing filter to reduce the glare from light reflections from the water surface, wet roads etc., the effect occurs in this likewise.

View of the sky

Particularly appealing is to observe the polarization tufts with the naked eye in the wild is. The basis of the Rayleigh scattering partially polarized, blue sky light provides a way to. When looking into the cloudless sky perpendicular to the sun, the colors used in the Haidinger - tuft from the trained observer can be clearly identified. Recommended is the observation during the construction or destruction of the sun in the sky area vertically above the observer (or along the arc South Zenit- Nord). You look relaxed for about a minute in the corresponding area of ​​the sky to let then the phenomenon are briefly visible through a subsequent rapid inclination of the head. The yellow arm of the tuft is always firmly based in this area of ​​the sky towards the sun, if you extended it as an arc of a great circle.

Consideration of reflections

Another possibility of observation in nature uses on a calm water surface reflected light of the cloudy sky. Just look to the method described above obliquely downward to the surface, for example a river, pond or a large rain puddle. Looking at the mirror image of the uniformly blue sky on a flat glass plate (eg window pane or the time switched off display of a tablet computer ) under the polarization angle ( Brewster angle ) is also to observe the phenomenon. The yellow arm appears here ( as in the reflection of water ) in the plane of incidence of the light.

Simulation of polarization tuft

People who are not able to see the Haidinger - tuft spontaneously, you can still get an impressive demonstration, by fixing the picture to simulate about 15 to 20 seconds, and keep their gaze fixed on it the center of the circle. This results in a negative afterimage, which can then be observed by turning away the look on a white background developed. First, the afterimage is a sharp complementary color version of the original, but during the fading, it deforms so that the two opposite areas of the colors on the center of the image appear to flow together. In this form, the further fading afterimage shows very close resemblance to the Haidinger - tufts. The intensity of the original image is obtained in the moment in which the first very clear afterimage appears only very weakly and soon disappears.