Yet another early method of producing polarized light used the phenomenon of the<br>Brewster angle of polarization mentioned at the end of the previous chapter. At the<br>Brewster angle for a given material, the reflected light becomes polarized in the plane<br>of the material’s flat surface. This is why unpolarized daylight becomes partly polarized<br>in the horizontal plane when it is reflected from a relatively flat horizontal surface<br>such as the sea. Sunglasses, fitted with vertically polarized filters, are used to reduce<br>the glare from horizontal surfaces, and this brings us to the use of such filters for gemmological<br>purposes.<br>The majority of polarizing filters today consist of a plastic sheet containing either<br>microscopic crystals of quinine idosulphate or, more recently, ‘long’ molecules. These<br>crystals and molecules are orientated so that they transmit light with minimum attenuation<br>only when it is vibrating in one plane, and progressively absorb those rays which<br>are polarized at increasing angles to this plane. Rays at 90° to this plane experie
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