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Polarized Light Microscopy
Interactive Tutorials
Nicol Prisms
Several versions of prism-based polarizing devices were once widely available, and these were usually named after their designers. The most common polarizing prism (illustrated in the tutorial window) was named after William Nicol, who first cleaved and cemented together two crystals of Iceland spar with Canada balsam in 1829. Nicol prisms were first used to measure the polarization angle of birefringent compounds, leading to new developments in the understanding of interaction between polarized light and crystalline substances. This interactive tutorial explores the generation of orthogonal or mutually perpendicular (ordinary and extraordinary) waves as the result of light transmission through a Nicol prism.
A typical Nicol prism is composed of a doubly refracting (birefringent) material, usually calcite, cut along the plane labeled a-b-c-d, as illustrated in the tutorial window. The two halves are then cemented together to reproduce the original crystal shape. A beam of unpolarized white light enters the crystal from the left and is split into two components that are polarized in orthogonal directions. One of these beams (labeled the ordinary ray) is refracted to a greater degree and impacts the cemented boundary at an angle that results in its total reflection out of the prism through the uppermost crystal face. The other beam (extraordinary ray) is refracted to a lesser degree and passes through the prism to exit as a plane-polarized beam of light. To operate the tutorial use the radio buttons labeled White Light and Monochromatic Light to toggle between these two illumination modes. When the tutorial is placed in monochromatic light mode, the Wavelength slider is activated, and can be utilized to alter the incident light wavelength in the range of 400 to 700 nanometers.
Polarized light microscopy was first introduced during the nineteenth century, but instead of employing transmission-polarizing materials, light was polarized by reflection from a stack of glass plates set at a 57-degree angle to the plane of incidence. Later, more advanced instruments relied on a crystal of doubly refracting material (such as calcite) specially cut and cemented together to form a prism. A beam of white unpolarized light entering a crystal of this type is separated into two components that are polarized in mutually perpendicular directions. One of these light rays is termed the ordinary ray, while the other is called the extraordinary ray. The ordinary ray is refracted to a greater degree in the birefringent crystal and impacts the cemented surface at the angle of total internal reflection. As a result, this ray is reflected out of the prism and eliminated by absorption in the optical mount. The extraordinary ray traverses the prism and emerges as a beam of linearly polarized light that is passed through the substage condenser directly to the specimen (positioned on the microscope stage).
Other prism configurations were suggested and constructed during the nineteenth and early twentieth centuries, but are no longer utilized for producing polarized light in most applications. Nicol prisms are very expensive and bulky, and have a very limited aperture, which restricts their use at high magnifications. Instead, polarized light is now most commonly produced by absorption of light having a set of specific vibration directions in a dichroic medium. Certain natural minerals, such as tourmaline, possess this property, but synthetic films invented by Dr. Edwin H. Land in 1932 soon overtook all other materials as the medium of choice for production of plane-polarized light. Tiny crystallites of iodoquinine sulfate, oriented in the same direction, are embedded in a transparent polymeric film to prevent migration and reorientation of the crystals. Land developed sheets containing polarizing films that were marketed under the trade name of Polaroid (a registered trademark), which has become the accepted generic term for these sheets. Any device capable of selecting plane-polarized light from natural (unpolarized) white light is now referred to as a polar or polarizer, a name first introduced in 1948 by A. F. Hallimond. Today, polarizers are widely used in liquid crystal displays (LCDs), sunglasses, photography, microscopy, and for a myriad of scientific and medical purposes.
Contributing Authors
Mortimer Abramowitz - Olympus America, Inc., Two Corporate Center Drive., Melville, New York, 11747.
Matthew J. Parry-Hill and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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