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WHAT IS DOUBLE REFRACTION ? WHAT IS NICOL PRISM? WHAT IS THE FUNCTION OF NICOL PRISM? WHAT IS THE CONSTRUCTION OF IT?

 Before knowing about double refraction, we need to understand refraction. So, refraction is the bending of light rays when it passes from one medium to another medium due to the different optical densities of the medium.

   To measure the optical density, a refractive index is used. The Refractive index of a medium may be defined as the ratio of the speed of light in a vacuum or air to the speed of light in that medium.

There are two cases obtained: 

1) if the incident angle and the angle refraction are zero and the refractive index of the first medium is not equal to the refractive index of the second medium, then the lights are not refracted.

2) If the refractive index of the first medium is equal to the refractive index of the second medium, then also the lights are not refracted.

Refraction follows some rules: 

1) The incident ray, refracted ray, and the normal at the point of incidence all lie in the same plane. 

2) The relation between the incident and refraction angle and the refractive indices of the medium, which is known as snell's law.

   It states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the refractive index of the second medium with respect to the first medium.



When an unpolarized light (vibrates all directions) strikes on a doubly refracting crystal-like calcite, quartz, etc. the unpolarized light splits into two refracted rays. One ray is called the “O” ray and the other is the “E” ray and this phenomenon is called double refraction. Both “O” and “E” rays are plane-polarized lights but, their direction of vibration of the electric field is different. The electric field of the “E” ray vibrates in the direction of the plane of the paper and the electric field of the “O” ray vibrates in the perpendicular direction of the plane of the paper.

NICOL PRISM

Nicol's prism is invented by William Nicol in the year 1828. It is an optical device, which is made from calcite crystal and is used in many optical instruments for producing and analyzing plane-polarized light.

      It is worked in the principle of double refraction.

After the phenomena of double refraction, the two refracted rays produce under the Nicol Prism one is the “O” ray, and another one is the “E” ray. The “O” rays obey the laws of refraction as well as laws of reflection but “E” rays do not follow any laws. Hence, the “O” rays are eliminated from the Nicol Prism due to total internal reflection phenomena and only “E” rays come out from the Nicol Prism. In this way, plane-polarized light in the direction of the plane of paper is produced from the Nicol Prism. 


Construction of Nicol Prism: 



It is constructed from the calcite crystal. At first, calcite crystal cuts into two prism shapes and connected them together with the help of a special type of cement, which is known as Canada balsam. It is a transparent material, which has a refractive index is about 1.55. Finally, the shape of the calcite crystal is Rhombohedral that means the angles P, Q, R, S are 112, 68, 112, and 68 degrees respectively as shown in the figure. The length of the Nicol prism is 3 times greater than its thickness.

   When unpolarized lights come and hit the Nicol prism, the lights split into two refracted rays. One ray is known as the “O” ray and the other is the “E” ray and this phenomenon is called double refraction. Both “O” and “E” rays are plane-polarized lights but, their direction of vibration of the electric field is different.

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