(a) Polaroids: Tourmaline crystal is a natural polarising material and is a semi-precious gem and also not available in large size. Polaroids are artificially made large sheets or plates and capable of producing a strong beam of polarised light.
In 1852, W.B. Herpath observed that small needle-shaped crystals of quinine iodosulphate now called Harpathite have the property of polarising light. But the size of these crystals is so small that they cannot be used for large scale polarisation of light. In 1932, E.H. Land invented the technique of aligning the harpathite crystals as long chains parallel to each other. He arranged a large number of these crystals with their axis parallel to one another and placed in between the sheets of plastic. Such sheets act even more efficiently than the tourmaline crystals and are called sheet polarisers or polaroids.
Polaroids can also be obtained from a sheet of polyvinyl by stretching these sheets, the molecules become parallel to the direction of stress. The sheet is then impregnated with iodine. This is called H-polaroid. On heating H-polaroids in the presence of a dehydrating agent, it becomes strongly dichoric. This is called k-polaroid.
The Polaroid sheets have a characteristic plane called transmission plane. When the axis of these two planes are parallel to one another the light transmitted by first is also transmitted by second Fig., but if the planes are perpendicular to each other, then light transmitted by first is blocked by second Fig. and no fight comes out.
Uses. Following are practical uses of polaroids:
(1) In sun glasses: Sun glasses fitted with polaroids instead of coloured glasses are more efficient in protecting the eyes from glare due to reflected fight without obscuring details even in shadows.
(2) Wind shield of automobiles: Wind screen fitted with polaroid as well as head lights of automobiles protect the eyes of the drivers of the automobile from the dazzling fight of approaching vehicles. The transmission axes of the polaroids both in the head fights as well as the wind screen are oriented say at 45° with the vertical.
(3) In window panes: For this purpose, two Polaroid sheets are mounted one behind the other. On rotating one of the polaroid sheet, the amount of fight entering the room can be controlled. This is generally used in trains or aeroplanes etc.
(4) Three dimensional motion pictures: To make stereoscopic motion pictures, two pictures of the same scene are taken at slightly different angles. These two pictures are projected simultaneously on the screen but fight in two pictures is polarised at right angle to each other. The spectators view these pictures with the help of polaroid spectacles, to have three dimensional effect.
(b) Optical rotation (or optical activity)
The property of the substances to rotate the plane of vibration of incident plane polarised light is called optical activity and such substances are called optically active substances.
There are two types of such substances:
(i) Dextro rotatory or right handed
The substances that rotate the plane of vibration to the right i.e. in clockwise direction are called dextro-rotatory or right handed, e.g. cane sugar solution, NaCl etc.
(ii) Laevo rotatory or left handed
The substances that rotate the plane of vibration to the left i.e. in anticlockwise direction are called laevo-rotatory or left handed, e.g. fruit sugar solution, turpentine, nicotine etc.
