Haidinger tufts
In 1844 Wilhelm Ritter von Haidinger pointed out a previously neglected ability of the human eye in the annals of physics : With the help of perception of an inconspicuous, colored appearance - later named after him as Haidinger tufts - most people are able to recognize the polarization state of visible light to some extent. It is possible for the experienced observer to differentiate between linearly or circularly polarized light, as well as to identify the polarization direction and estimate different degrees of polarization.
The Haidinger tuft is an entoptic phenomenon, which means that it only arises in the eye of the observer and therefore cannot be photographed .
Description of the phenomenon
An observer looks calmly for a few seconds in a direction from which linearly polarized light hits the eye. If he then tilts his head to one side - still looking in the direction - the so-called Haidinger tuft appears briefly. If you then tilt your head alternately to the left and right, the phenomenon can be re-created.
If linearly polarized light with a constant direction of polarization hits the eye, this light is not of natural, i.e. H. not to distinguish polarized light. However, if the direction of the polarization changes abruptly with respect to the eye and then remains constant, the polarization bundle appears for a few seconds, only to fade again shortly afterwards ( similar to an afterimage ). In the event that the direction of polarization rotates continuously, the Haidinger tuft is also permanently visible. It then does not fade and rotates in the same direction as the polarization.
Observers describe the Haidinger tuft as a diffuse, yellowish shape, which is constricted in the middle and is cut vertically by a corresponding bluish-violet shape in the middle (similar to a four-leaf clover). The appearance is very inconspicuous, which is why a plain background without distracting patterns is recommended for observation. The two perceptible colors (bluish-purple and yellow) are complementary colors . Depending on the observation situation, it can be different for one and the same viewer whether he sees the yellow or the bluish arm of the tuft as a continuous stripe.
If the observer waits long enough for the appearance to fade and then averts his gaze to an unpolarized source, a negative afterimage will appear briefly, with the former yellow arm now blue and vice versa.
The orientation of the blue double bundle corresponds to the polarization direction of the linearly polarized light hitting the eye.
The Haidinger tuft encompasses a viewing angle of approximately 3 ° to 4 °, that is, the expected diameter corresponds approximately to the width of two fingers placed next to one another, which are viewed at a distance from the outstretched arm. Maxwell gave a first hint as to the obvious cause of the phenomenon. To him, "the area covered by the polarization figure appeared to be equal in size to the vessel-free area of the yellow spot ."
In order to be able to perceive the tuft, the degree of polarization of the incident light must be at least 60% and - as Stokes found out - blue , polarized light (wavelengths less than 500 nm; also contained in white light) must necessarily fall on the eye. The effect appears intensified in pure blue light, yellow is then replaced by a dark blue sensory impression.
With clockwise circular polarization , the (upright) observer in the left and right eye sees the yellow tuft with its longitudinal axis aligned from top right to bottom left, at an angle of approximately + 45 °. The image is fixed in relation to the retina and only rotates when the observer tilts his head. With left-handed polarization, the angle is approx. −45 °, so that the left and right figures appear rotated by 90 ° to each other. The orientation can differ slightly between the right and left eye.
Formation of the Haidinger tuft
The results of previous research cannot yet explain all aspects of the phenomenon. Physiologists see the cause of the phenomenon mostly in the radial arrangement of the nerve fibers diverging from the fovea centralis in combination with the pigmentation ( xanthophylls ) in the yellow spot ( macula ). This combination acts like a radially symmetrical polarization filter , i. H. Depending on the direction of polarization of the light falling on the fovea, only certain areas are excited and thus cause an effect that is perceptible to humans.
Due to their rod-shaped structure, xanthophyll molecules have a strongly anisotropic behavior and are therefore sensitive to the polarization of light. An electromagnetic wave hitting a molecule can stimulate both electrical states and oscillation states in this molecule, especially when the direction of the electric field component of the light wave (polarization direction) is parallel to the longitudinal axis of the molecule.
The illustration opposite shows the pigment molecules arranged in concentric circles. The xanthophyll molecules (in section B), aligned parallel to the polarization direction of the incident light, absorb most of the light. In sector A it is exactly the opposite. This creates the typical shape of the polarization bundle. In this case, area B appears yellow and A blue (complementary color).
The image then fades due to fatigue and the associated development of a negative afterimage (due to the entoptic nature of the effect), which lies exactly over the original. Because the effect is weak, the two images wash each other out unless the polarization is suddenly eliminated, whereupon only the afterimage is visible. If the plane of polarization is suddenly rotated by 90 ° or the direction of rotation (of the circular polarization) is suddenly changed, the afterimage will intensify the refreshed image.
Use in ophthalmology
The Haidinger tuft always appears exactly in the direction in which the foveola is pointing and is only recognized by this. In the case of eccentric fixation , the tuft is perceived next to the fixation point. Ophthalmology makes use of this property of the phenomenon in the fixation test and in pleoptic exercise treatments to train foveolar perception.
Special ophthalmic devices, so-called haploscopes , can make the Haidinger tufts even more recognizable by using rotating polarization filters and an additional cobalt blue filter. In this case it appears as a blue vortex or " propeller ". The German eye doctor Curt Cüppers is considered to be the inventor of this examination arrangement .
Observation possibilities
Many people have difficulty recognizing the Haidinger tuft on the first try. The actual quality of its appearance is much weaker than in pictures, and it has a tendency to appear and disappear. The famous German physiologist and physicist Hermann von Helmholtz commented: “Even 12 years ago, immediately after Haidinger's discovery, with the greatest difficulty, I was unable to perceive anything of the tufts, and recently, when I tried again, I saw them the first View through a Nicolian prism . "
The following methods sometimes use technical aids, but they only serve to present linearly polarized light to the eye . If you train for a few minutes several times a day at the beginning, after one or two days the familiar figure of the Haidinger tuft can be recognized without much effort.
Liquid crystal screens
LC displays emit linearly polarized light, which enables uncomplicated observation of the phenomenon in a well-darkened room. If you fix your eye on a point on a white surface on the screen (for example an empty browser window), the Haidinger tuft can usually be recognized (after a short period of getting used to the delicacy of the appearance). It is advisable to start at a distance of about 50 cm, as the tuft is easier to find because of its size. The head is tilted to the side (placed on the shoulder) in order, for a few seconds, to contemplate the white surface. The tuft only appears if you now lay your head on the other side and continue to look at the screen. The diagonal position of the tuft can be observed (many displays are diagonally polarized to make it easier to use with vertically polarizing sunglasses ) and the change in size when the distance between the eye and the monitor is varied. With diagonal polarization of the display, the Haidinger tuft also appears diagonally in front of the viewer's eye. Viewing the empty, white color LC display, for example on a mobile phone or tablet computer, is easier because instead of the head, only the display itself has to be rotated and the polarization cluster remains permanently visible as the rotation continues.
Polarizing filter
The phenomenon appears when looking through a linear polarization filter onto a bright, white surface (for example an illuminated sheet of paper, white cloud). If you turn the filter, you change the direction of polarization of the light hitting the eye continuously and so the tuft, which otherwise only appears briefly, rotates in the same direction and is easier and permanent to perceive through this movement. Since many sunglasses contain vertically polarizing filters to reduce the glare caused by light reflections from water surfaces, wet roads, etc., the effect also occurs with these.
Look at the sky
It is particularly attractive to observe the polarization bundle with the naked eye in the wild. The blue light from the sky , partially polarized due to Rayleigh scattering , offers a possibility for this. When looking at the cloudy sky perpendicular to the sun , the colored appearance of the Haidinger tuft can be clearly identified by the experienced observer. It is advisable to observe during the rising or setting of the sun in the sky vertically above the observer (or along the arc south – zenith – north). You look relaxed for about a minute into the corresponding area of the sky, and then quickly tilt your head to make the phenomenon visible for a short time. In this area of the sky, the yellow arm of the tuft is always firmly oriented towards the sun if it is extended as an arc of a great circle .
Viewing reflections
Another possibility of observation in nature uses the light of the cloudy sky reflected on the calmest possible water surface . To do this, use the method described above to look diagonally downwards at the surface of a river, pond or a large rain puddle, for example. If you look at the reflection of the evenly blue sky on a flat glass plate (e.g. window pane or the display of a tablet computer , which is switched off this time ) at the angle of polarization ( Brewster's angle ), the phenomenon can also be observed. The yellow arm appears here (as with the reflection on water) in the plane of incidence of light.
Simulation of the polarization bundle
People who are unable to see the Haidinger tuft spontaneously can still get an impressive demonstration by fixing the image for about 15 to 20 seconds to simulate, while keeping their gaze fixed on the center of the circle. A negative afterimage develops , which can then be observed by turning your gaze to a white background. At first, the afterimage is a sharp complementary color version of the original, but as it fades it deforms so that the two opposite areas of one of the colors appear to flow together over the center of the image. In this form, the afterimage, which continues to fade, is very similar to the Haidinger tuft. The intensity of the original is reached at the moment when the initially very clear afterimage appears only very weak and disappears shortly afterwards.
literature
- Marcel Minnaert : light and color in nature. Birkhäuser Verlag AG, Basel et al. 1992, ISBN 3-7643-2496-1 .
- Marcel Minnaert: Haidinger tufts. In: Friends of Goethean Color Theory. Spectrum of clubs. Issue 4, 2002/2003, pp. 18-23, online (PDF; 436 kB) .
- Albert Pröbstl: The Haidinger tuft as a primordial phenomenon of polarization phenomena. In: Elements of Science. Vol. 69, No. 2, 1998, ISSN 0422-9630 , pp. 1-26.
- Herbert Kaufmann (Ed.): Strabismus. 3rd, fundamentally revised and expanded edition. Thieme, Stuttgart et al. 2004, ISBN 3-13-129723-9 (on the use of the Haidinger tuft in ophthalmology).
Web links
- Johannes Grebe-Ellis: To the Haidinger tuft. In: Volker Nordmeier (Red.): Didaktik der Physik - Leipzig 2002. Contributions to the spring conference of the DPG Leipzig 2002. Lehmanns, Berlin 2002, ISBN 3-936427-11-9 , (PDF file; 631 kB).
- Herbert Kaufmann (Ed.): Strabismus. 3rd, fundamentally revised and expanded edition. Thieme, Stuttgart et al. 2004, ISBN 3-13-129723-9 , p. 269 (on the use of the Haidinger tuft in ophthalmology).
- www.polarization.com/haidinger (English)
- Macula Integrity Tester ™ as an example of a device for testing and training eccentric fixation (commercial website, English)
Individual evidence
- ^ Wilhelm Haidinger: About the direct recognition of polarized light and the position of the plane of polarization. In: Annals of Physics. Volume 63, No. 1844, ISSN 0003-3804 , pp. 29-39. (Original contribution to the Gallica digitization project at the Bibliothèque nationale de France ).
- ^ A b c d Maxwell B. Fairbairn: Physical Models of Haidinger's Brush. In: Journal of the Royal Astronomical Society of Canada . Volume 95, No. 6, December 2001, ISSN 0035-872X , pp. 248-251 (p. 248).
- ↑ a b Hermann Helmholtz: Handbook of physiological optics. (= General Encyclopedia of Physics. Vol. 9). Text volume Leipzig, Leopold Voss 1867, p. 421 ff.
- ^ Richard A. Bone: The role of the macular pigment in the detection of polarized light. In: Vision Research , Vol. 20, No. 3, 1980, pp. 213-220, doi: 10.1016 / 0042-6989 (80) 90105-4 .
- ↑ US Patent US3044348 A - Haidinger's brush attachment for synoptophore apparatus