Nicol prism

A Nicol prism is a birefringent optical component made from calcite (calcium carbonate crystal) that splits unpolarized light into two orthogonal polarization states and uses internal reflection to transmit only one of them. The device consists of two pieces of optically clear calcite cut at specific angles, cemented together with Canada balsam, and mounted in a brass or aluminum tube. The light path through the prism is bent such that one polarization component (the ordinary ray) undergoes total internal reflection at the interface between the calcite and balsam, bouncing out through the side of the prism as waste heat or stray light, while the other component (the extraordinary ray) passes through and emerges as linearly polarized light.

William Nicol, a Scottish physicist, patented this design in 1828, and it became the standard tool for generating and analyzing polarized light throughout the nineteenth century and well into the twentieth. The prism's geometry is critical: calcite is cut at roughly 68 degrees to the optical axis so that the refractive indices of the two rays differ enough to create the necessary reflection angle. Typical prisms are 1 to 2 centimeters in length and pass wavelengths from ultraviolet through the visible spectrum into the near infrared, though transmission drops sharply outside the visible range.

Why it mattered, and why it is now obsolete

Before the advent of synthetic polarizing films and coated dichroic mirrors, Nicol prisms were the only reliable way to generate high-quality polarized light in a laboratory. Optical benches, polarimetry instruments, spectrophotometers, and photographic equipment all relied on Nicol prisms to control light polarization. Because calcite is birefringent, the prism also found use as a polarizing beam splitter, dividing an input beam into two perpendicular polarization channels for interferometry and other precision work.

The emergence of H-sheet (dichroic) polarizers in the 1930s and 1940s, followed by modern thin-film and polymer-based polarizing films, rendered the Nicol prism largely obsolete for routine laboratory work. These alternatives are cheaper, more compact, have better extinction ratios at visible wavelengths, and suffer fewer losses in the infrared. However, Nicol prisms still appear in specialized applications: some vintage optical instruments remain in service, and the prism remains a teaching tool because its operation illustrates birefringence and optical anisotropy in a way no film can match.

Practical limitations of the Nicol prism include sensitivity to misalignment (which degrades polarization purity), chromatic effects (the extinction ratio varies with wavelength), and the brittleness and cost of large, optically homogeneous calcite crystals. Canada balsam, the cement, can yellow and deteriorate over decades, reducing transmission. The device also introduces astigmatism and other aberrations because light exits at an angle to the optical axis, a problem that later variants (such as the Glan-Thompson prism, which used a different cement) tried to minimize.