Double Refraction:
Nicol prisms are made from a birefringent material, typically calcite (calcium carbonate). Birefringence is the optical property of a material that causes light to split into two rays when passing through it. In calcite, the incident light is divided into two rays—the ordinary ray (o-ray) and the extraordinary ray (e-ray).
Construction of Nicol Prism:
A Nicol prism is constructed by cutting a calcite crystal diagonally with a specific angle and then cementing the two halves back together with Canada balsam, a resin with a refractive index close to that of calcite. This creates a boundary between the two halves of the prism.
Polarization by Nicol Prism:
When unpolarized light enters the Nicol prism, it undergoes double refraction, and the o-ray and e-ray travel through the prism with different velocities and in different directions. The refractive index of the extraordinary ray is lower than that of the ordinary ray in calcite, causing the e-ray to be bent more significantly at the interface of the two halves of the prism.
Total Internal Reflection:
The cemented interface between the two halves of the Nicol prism is designed to be at a specific angle known as the Brewster angle (about 68 degrees for calcite). At this angle, the e-ray undergoes total internal reflection, meaning it is completely reflected back within the prism and does not emerge from the other side.
Polarized Output:
On the other hand, the o-ray, which experiences a smaller change in direction, continues through the prism. As a result, only the o-ray emerges from the Nicol prism as polarized light. This polarized light vibrates in a single plane perpendicular to the extraordinary ray.
Using Nicol Prism as a Polarizer:
By placing a Nicol prism in front of a light source or optical system, it can be used as a polarizer. It allows polarized light, specifically the o-ray, to pass through, while blocking the e-ray due to total internal reflection. This results in the production of linearly polarized light.
In conclusion, the Nicol prism, through the principles of double refraction and total internal reflection, functions as a polarizer by selectively transmitting one of the two orthogonally polarized rays of light, thus providing polarized output.
