Wave Optics — Physics STD 12 Science — Question

Polaroid : It is a cheap and commercial device for producing plane polarised light. Herapathite is used as raw material in the manufacture of polaroids (by Polaroid Corporation in America). There are a number of methods of preparing polaroid sheets. One method, is said to consist of first breaking up the mineral herapathite into needle like microcrystals and forming a suspension of these in introcellulose. This is then forced through a horizontal slit and the thin sheet coming through the slit is mounted between two thin sheets of glass.

In a polaroid film, all the dichroic crystals are arranged in such a way that the optic axes of all the crystals are parallel. These crystals are highly dichroic, absorbing one of the doubly-refracted beams completely. Each polaroid film has a characteristic direction which is called the 'polarising direction'. In adjacent Fig., this direction is shown by parallel lines.
Uses of Polaroid : (i) One of the main uses of polaroids is to avoid the glare of light. The light reflected from bright surfaces such as wet roads, cover glasses of paintings, polished tables, glazed and white paper, is partially plane-polarised with vibrations in the horizontal plane. This light on reaching the eye produces glare. (It is for this reason that our eye experience glare while reading a book of glazed paper. The glare can be avoided by wearing polarised sun-glasses with their vibration-planes vertical. Then most of the polarised light reflected from the objects will be cut-off. (We see the objects in diffused light.)
(ii) Polaroids are used in motor-cars to avoid the dazzling light of a car approaching from the opposite direction. For this, the polaroids are fitted on the cover glasses of the head-light and the wind screen of the vehicle.
(iii) Sometimes we are not able to see properly very minute particles through a microscope due to glare. Then microscopes fitted with polaroids are used.
(iv) Distinct photographs of clouds can be taken by fitting polaroids in front of the camera-lens. Generally, since the scattered light is always present in the atmos-phere, we cannot take distinct photograph of clouds due the light reflected from the clouds. Since the scattered light is partially-polarised, it is mostly cut off by the polaroid fitted in the camera; while the light reflected from the clouds, being unpolarised, enters the camera. Thus, the background becomes sufficiently dark against which a clear photograph of white clouds is obtained.
(v) Polaroids are used to control the intensity of light entering trains and aeroplanes. For this, one polaroid is fitted outside the window and the other is fitted inside which can be rotated. The intensity of light can be adjusted by rotating the inner polaroid.
(vi) Polaroid glasses are used for viewing three-dimensional pictures.
(vii) Polaroids are used in studying optical properties of metals and in analyzing crystals.
(viii) Polaroids can be used to distinguish between unpolarised, partially plane polarised and completely plane polarised light with the help of a polaroid.