Polarized light

Although an object seen through something transparent normally appears as a single image, it is not always the case. In 1669, Erasmus Bartholin described how single crystals of Icelandic spar (calcite) produced a double image. He rotated the crystal and found that one image moved whilst the other stayed in the same position. This occurs when light from an object meets the calcite and is refracted because it is passing from one medium to another, but the refraction takes place in an unusual way. Waves that are moving on one particular plane are bent by a different amount than waves moving at right angles to them so two sets of light rays are produced. This splitting of light is known as double refraction or birefringence and the emerging light as polarized light. In 1808 Etienne Malus looked through Icelandic spar under reflected light and found that one of the images had disappeared. He concluded that ordinary daylight was made of two forms of light which the crystal bent in different ways. At a certain angle only one form of light was reflected by the mirror so only one image could be seen. It is now know that the difference between these forms of light lies in their polarity or nature of their waves, Daylight Is usually unpolarized; its waves move up and down at all angles to the direction of its movement. Reflected light is partially polarized, i.e. its waves move mainly in one plane. French statesmen and scientist Francois Arago followed the work of Malus with his own studies on the nature of polarized light. He investigated the polarity of light from different parts of the sky and in 1812 he built one of the first polarizing filters which he made from a stack of glass sheets.