Brewster, David (1781-1868)

Scottish physicist who made discoveries about the diffraction and polarization of light, and invented the kaleidoscope 1816. Brewster was born in Jedburgh. Although he never took his degree, he was made principal of Edinburgh University 1859. In 1813 Brewster was able to demonstrate, by studying the polarization of light passing through a succession of glass plates, that the index of refraction of a particular medium determines the tangent of the angle of polarization for light that transverses it. Brewster then sought an expression for the polarization of light by reflection and found, in 1815, that the polarization of a beam of reflected light is greatest when the reflected and refracted rays are at right angles to each other. This is known as Brewster's law: the tangent of the angle of polarization is numerically equal to the refractive index of the reflecting medium when polarization is maximum. Brewster then worked on the polarization of light reflected by metals, and established the new field of optical mineralogy.