- In the case of some crystals like calcite or quartz, a single ray of unpolarized light from a point source of light splits up into two refracted rays casting two images of the point object. This phenomenon is known as double refraction or birefringence.
- In this case, both the refracted rays are polarized in two mutually perpendicular planes.
- The phenomenon of double refraction is used to get polarized light from Nicol prism. This was constructed by William Nicol in 1828 A.D. In this method ordinary rays are eliminated by total internal reflection and only extraordinary rays are allowed to come out of the prism.
A naturally occurring form of calcium carbonate is known as Calcite or Iceland spar. In Nicol’s prism, the length of the crystal is three times the breadth. The ends are grounded until they make an angle of 68° and 112° to form a parallelogram. These crystals are cut into two halves and cemented together by means of Canada balsam. This gives Nicol prism.
- Nicol’s prism can be used as polarizer as well as an analyzer.
- Polaroid is a large sheet of synthetic material packed with tiny crystals of a dichroic substance oriented parallel to one another so that it transmits light only in one direction of the electric vector.
- Doubly refracting crystals split the incident unpolarized light passing through them into O – light, and E – light which are plane polarized in mutually perpendicular planes.
- Some such crystals have the property of absorbing the ordinary and extra ordinary light unequally. This property is called dichroism and crystal possessing this property are called dichroic. e.g. tourmaline. It absorbs O – light and transmits E – light.
- Polaroids are artificially prepared dichroic substances. In 1852, W. H. Herapath discovered synthetic crystalline material iodosulphate of quinine known as herapathite which exhibits dichroism.
- In 1934 E.H. Land embedded tiny herapathite crystals in cellulose acetate with their optic axis parallel. The layers of the crystal were mounted between two glass sheets for protection. This act as a sheet of the polarizer and called as polaroids.
- It can be used as polarizer as well as an analyzer.
Uses of Polaroids:
- Polaroids are used in sun glasses to cut off the glare reflected by horizontal surfaces.
- Polaroids are used in glass windows of an airplane to control the intensity of light entering the airplane.
- Polaroid glasses are used to view three-dimensional pictures and movies.
- Polaroids are used as a filter in the photographic camera.
- They are used in production and analyzation of plane polarized light.
- They are used to improve colour contrast in old oil paintings.
- They are used in calculators, watches, monitors of laptops which have LCD screens.