In 1982, W.B.Herapath,an English physical developed dichronic crystals of the organic compound quinie iodosulphate called herapathite. Such a crystal completely absorbs one component of polarization and transmits the order with little loss. Such a crystal completely absorbs one component of polarisation and transmits the other with little loss. These crystals are capable of transmitting linearly polarized light of all colours. However these crystals are very small in size and cannot be used singly for producing or analysing polarised light.
In 1932.E.H.Land developed large size polarizing films mounted between two thin plates of optical glass. These films consist of thin sheets of nitrocellulose packed with ultramicroscopic crystals of herapthine with the optic axes of all parallel so that they function as single crystal. The film mounted between two thin plates of optical glass is called Polaroid.
The most common types of Polaroid used today are H-sheet and K-sheet which were developed bty E.H.Land in 1938. H-polaroid with a film of polyvinyl alcohol (PVA) which is stretched to line up the complex molecules and then impregnated with iodine. The film is then mounted between two thin glass plates. K-polaroid is prepared by heating an oriented transparent of PVA in the presence of an active dehydrating catalyst such as hydrogen chloride. The films darkens slightly and becomes strongly dichroic. Such a film becomes very stable and, having no dyestuffs, is not bleached by strong sunlight.
When two polaroids are arranged with their axes parallel, one component (O-ray) of polarised light is absorbed while the other (E-ray) is transmitted.
When they are arranged with their axes perpendicular to each other (crossed position), the second film absorbs completely the light coming from the first
Use of polaroids
Polaroidss are used to produce and analyse plane polarised light. Because of low cost, polaroids are used instead of nicol prisms, in modern polarizing instruments. They are used to produced non-glare spectacles. They are used in the windows of trains, aeroplanes, etc. for controlling the amount of light entering from outside. They are also used in the study of optical properties of metals.
Polaroids are used in photography for preventing the unwanted glare from surface such as cover glasses of paintings, polished tables and shining metallic surfaces. They are used to improve colour contrast in old oil paintings.
Polaroids are used for the production of non-glare headlights and wind shields of automobiles. If the headlights and wind shields of all automobiles are equipped with K-polaroids with their vibration planes inclined at 45degree with the horizontal, the transmission planes of the wind shields of approaching vehicles will become at right angles to the incident light. So the wind shield eliminates or considerably reduces the intensity of the incoming light..
Stereoscopic motion pictures make use of polaroids. Two views of the same scene are made simultaneously. one corresponding to the view seen by the right eye and the other by the left eye. T he two views are projected on a special type of screen through two separate polaroids with their vibration planes at right angles to each other. If an observer sets the superimposed pictures through polaroid spectacles such that the vibration plane of the polaroid over his right eye is parallel to the vibration plane of the corresponding projected picture and the vibration plane of the polaroid over his left eye is parallel to the vibration plane of the other projected picture, the scene appears to be in three dimension