Polaroids And Optical Rotations
A polaroid is a material which polarizes light. Tourmaline is a natural polarizing material. Polaroids are now artificially made. It was discovered that small needle shaped crystals of quinine idosulphate have the property of polarizing the light. A number of these crystals with their axes parallel to one another are packed in between two sheets of plastic. Such a sheet serves as the polaroid.
Uses of plane polarized light and polaroids
1. One of the major uses of Polaroids is to avoid glare of light. Most of the light reflected from glazed surfaces is partially plane polarized with vibrations in the horizontal plane. When we use planes-vertical, the most of the polarized light reflected from glazed surfaces is cut off.
2. To avoid the dazzling light of a car approaching from the opposite side during night driving, polaroids are fitted on the wind shield and on the cover glasses of head lights of each car. The arrangement is so made that polaroids on the wind shield of one car and those on cover glasses of head lights of other car become crossed. Therefore, the glare is avoided. However, each driver can see the other car from the head lights of his own car.
3. The microscopes objectives are fitted with polaroids to avoid glare in observing very minute particles.
4. Clear photographs of white clouds are obtained by fitting polaroids in front of the camera lens. Scattered light present in the atmosphere, which is partially polarized is cut off by the polaroid. Only the light reflected from the clouds, being unpolarized, enters the camera and we obtain the distinct picture of clouds.
5. Polaroids are useful in three dimensional motion pictures, i.e. in holography.
6. Polaroids are used to improve colour contrast in old paintings.
7. Polaroids are also used in optical stress analysis.
8. In calculators and watches, letters and numbers are formed by liquid crystal display (LCD) through polarization of light.
9. In CD players, polarized laser beam act as needle for producing sound from compact disc, which is in encoded digital format.
10. Polarization of scattered sunlight is used for navigation in solar compass in polar regions, where magnetic compass becomes inoperative.
11. By determining the polarizing angle (ip) and using Brewster’s law, refractive index of dark transparent substances can be determined.
12. Polarization is also used to study asymmetries in crystals and molecules using the phenomenon of optical activity.
Optical rotation is the phenomenon of rotating the plane of polarization of light about the direction of propagation of light, when passed through certain crystals or solutions.
For example, monochromatic light from a source S is incident on a polarizer P. the plane polarized light from P is made to fall on an analyser A, which is crossed with P. no light comes out of A. when the quartz plate is inserted between the crossed nicols P and A, some light emerges out of the analyser. The emerging light is cut off again when analyser A is rotated further through a certain angle.
This shows that light emerging from quartz plate is still plane polarized, but its plane of polarization has been rotated through a certain angle. This angle is equal to the angle through which analyser A is to be rotated to cut off the light again.
The substances which rotate the plane of polarization are said to be optically active. Examples of optically active substances are: quartz, sugar crystals, turpentine oil, sodium chloride etc. such crystals can be divided into two types:
(i) Dextro rotator or right handed, which rotate the plane of polarization in the clockwise direction.
(ii) Laevo rotator or left handed, which rotate the plane of polarization in the anticlockwise direction.
The angle of rotation depends on
(i) Thickness of crystal,
(ii) Density of crystal or concentration in case of solutions,
(iii) Wavelength of light used,
(iv) Temperature of the solution.
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