Question

Explain the construction and working of Nicol prism

Answer 1

Nicol prism is a type of polarizer, an optical device made from calcite crystal used to produce and analyse plane polarized light. It is made in such a way that it eliminates one of the rays by total internal reflection, i.e. the ordinary ray is eliminated and only the extraordinary ray is transmitted through the prism. It was the first type of polarizing prism, invented in 1828 by William Nicol (1770–1851) of Edinburgh. It consists of a rhombohedral crystal of Iceland spar (a variety of calcite) that has been cut at an angle of 68° with respect to the crystal axis, cut again diagonally, and then rejoined as shown, using a layer of transparent Canada balsam as a glue.[1]

Unpolarized light ray enters through the left face of the crystal, as shown in the diagram, and is split into two orthogonally polarized, differently directed rays by the birefringenceproperty of the calcite. The ordinary ray, or o-ray, experiences a refractive index of no = 1.658 in the calcite and undergoes total internal reflection at the calcite–glue interface because its angle of incidence at the glue layer (refractive index n = 1.55) exceeds the critical angle for the interface. It passes out the top side of the upper half of the prism with some refraction, as shown. The extraordinaryray, or e-ray, experiences a lower refractive index (ne = 1.486) in the calcite and is not totally reflected at the interface because it strikes the interface at a sub-critical angle. The e-ray merely undergoes a slight refraction, or bending, as it passes through the interface into the lower half of the prism. It finally leaves the prism as a ray of plane-polarized light, undergoing another refraction, as it exits the far right side of the prism. The two exiting rays have polarizations orthogonal (at right angles) to each other, but the lower, or e-ray, is the more commonly used for further experimentation because it is again traveling in the original horizontal direction, assuming that the calcite prism angles have been properly cut. The direction of the upper ray, or o-ray, is quite different from its original direction because it alone suffers total internal reflection at the glue interface, as well as a final refraction on exit from the upper side of the prism.

Nicol prisms were once widely used in microscopy and polarimetry, and the term "using crossed Nicols" (abbreviated as XN) is still used to refer to the observing of a sample placed between orthogonally oriented polarizers. In most instruments, however, Nicol prisms have been replaced by other types of polarizers such as Polaroid sheets and Glan–Thompson prisms.

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Answer 2

Nicol prism is mainly used for the purpose of producing as well as analyzing plane polarised light.

Double refraction is the phenomenon associated with it.

Construction:

The base material is calcite crystal( ABCD).

Length is three times that of breadth.

End faces - AB and CD cut in a way that the principal section angles come to 68° and 112°.

Then, crystal is cut in a diagonal manner generating two parts; surfaces made flat and polished nicely.

Polished surfaces are then joined with Canada Balsam( a type of special cement).

Working:

A beam of unpolarized light falls on a surface of the prism and gets divided into two refracted rays.

These refracted rays are plane and polarized in nature; vibrations form right angle with each other.

For extraordinary ray of light: Canada Balsam is an optically denser medium.

For ordinary ray of light: Canada Balsam is an optically rarer medium.

When this ray falls on the prism and passes through this particular layer, it is a transition from a denser to a rarer medium.

Since the incident angle is much greater than the critical angle in this case, there is total internal reflection of the incident ray.

The extraordinary ray ,however, passes through it.