A comment on Licht und Stoff by Mario Howald-Haller
Optical polarisation is usually regarded as that area of classical optics in which the transverse structure of the field of light waves can be experimen- tally demonstrated in a particularly convincing way. This demonstration, although described as experimental, is, strictly speaking, produced medi- ately and not based on direct observation. This is indeed historically com- prehensible, but in the 20th century it is increasingly seen as a fundamental deficiency in physical optics. Thus the assertion that light itself is invisible, yet bestows visibility in its characteristically different ways, has, over the entire field of optics, led to the quest for observation-based formulations of optics as phenomenology. In his article Licht und Stoff, Mario Howald-Haller has drawn attention to the surprisingly simple and immediately perception-related way into the field of optical polarisation offered by the phenomenon of Haidinger‘s brush. In the context of optics this is the hitherto little known contrast phenomenon, in which the human eye, exposed to linear polarised light, answers with a kind of after-image. For the duration of a few seconds a very delicate, complementary coloured and cruciformly symmetrical pattern is visible whose orientation clearly indicates the direction of polarisation of the incident light. Consequently, through the identification of this phenomenon, called ‘Haidinger‘s brush’ after its discoverer, polarisation can be seen directly: in the double images of calcite (birefringence), in the mirroring of the surface of still water (Brewster angle) and in the blue of the sky (Rayleigh scattering).