A Nicol prism is a type of

polarizer A polarizer or polariser is an optical filter that lets light waves of a specific polarization (waves), polarization pass through while attenuation, blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed ...

. It is an optical device made from

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

crystal used to convert ordinary light into 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 Edinburgh is the capital city of Scotland and one of its 32 Council areas of Scotland, council areas. The city is located in southeast Scotland and is bounded to the north by the Firth of Forth and to the south by the Pentland Hills. Edinburgh ...

Mechanism

The Nicol prism consists of a

rhombohedral In geometry, a rhombohedron (also called a rhombic hexahedron or, inaccurately, a rhomboid) is a special case of a parallelepiped in which all six faces are congruent rhombus, rhombi. It can be used to define the rhombohedral lattice system, a Ho ...

crystal of Iceland spar (a variety of

) that has been cut at an angle of 68° with respect to the crystal axis, cut again diagonally, and then rejoined, using a layer of transparent Canada balsam as a glue. Unpolarized light ray enters through the side face of the crystal, and is split into two orthogonally polarized, differently directed rays by the birefringence property of calcite. The ''ordinary'' ray, or ''o''-ray, experiences a

refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

of ''n''_o = 1.658 in the calcite and undergoes a total internal reflection at the calcite–glue interface because of its angle of incidence at the glue layer (refractive index ''n'' = 1.550) exceeds the critical angle for the interface. It passes out the top side of the upper half of the prism with some

refraction In physics, refraction is the redirection of a wave as it passes from one transmission medium, medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commo ...

. The ''extraordinary'' ray, or ''e''-ray, experiences a lower refractive index (''n''_e = 1.486) in the calcite crystal 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

, 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 opposite side of the prism. The two exiting rays have polarizations

orthogonal In mathematics, orthogonality (mathematics), orthogonality is the generalization of the geometric notion of ''perpendicularity''. Although many authors use the two terms ''perpendicular'' and ''orthogonal'' interchangeably, the term ''perpendic ...

(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.

Uses

Nicol prisms were once widely used in mineralogical

microscopy Microscopy is the technical field of using microscopes to view subjects too small to be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of microscopy: optical mic ...

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.

References

{{reflist 1828 in science Microscopy Optical materials Polarization (waves) Prisms (optics) Scottish inventions