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SM195     Norremberg’s Apparatus for polarisation of light     circa 1940     Light

  When a ray of light falls on a polished, unsilvered glass surface at an angle of 35o 25′, that is when the angle of incidence is 54o 35′, it is reflected and the resultant ray is polarised in the plane of reflection.   If the angle of incidence differs from this, the resultant ray is only partially polarised.   This angle differs for various substances and the phenomenon occurs where there is both reflection at the surface and refraction through the medium.   Brewster discovered a simple law in reference to the polarising angle:

  The polarising angle of a substance is that angle of incidence for which the reflected polarised ray is at right angles to the refracted ray.

  Norremberg’s apparatus is used to polarise light by reflection.   Two brass rods support an unsilvered mirror that can rotate with respect to the incident light.   Between the feet of the brass rods is a mirror of silvered glass that is fixed and which transmits the polarised light vertically upwards, passing through an aperture on top, where it meets a black glass inclined to it at the polarising angle.   This can be rotated in a horizontal plane until a position is found where the image in the black glass is cut off.   Placing slides containing materials that rotate the direction of polarisation (many transparent minerals) between the crossed polarising mirrors produces some interesting effects when the resultant ray of light is viewed.