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Polaroid is the name of a type of synthetic plastic sheet which is used to polarize light. The...

Polaroid is the name of a type of synthetic plastic sheet which is used to polarize light. The original material, patented in 1929 (U.S. Patent 1,918,848) and further developed in 1932 by Edwin H. Land, consists of many microscopic crystals of iodoquinine sulfate (herapathite) embedded in a transparent nitrocellulose polymer film. The needle-like crystals are aligned during manufacture of the film by stretching or by applying electric or magnetic fields. With the crystals aligned, the sheet is dichroic: it tends to absorb light which is polarised parallel to the direction of the crystal alignment, but transmits light which is polarised perpendicular to it. The resultant electric field of an electromagnetic wave (such as light) determines its polarisation. If the wave interacts with a line of crystals as in a sheet of polaroid, any varying electric field in the direction parallel to the line of the crystals will cause a current to flow along this line. The electrons moving in this current will collide with other particles and re-emit the light backwards and forwards. This will cancel the incident wave causing little or no transmission through the sheet. The component of the electric