Polarization of Light
Sunlight and almost every other form of natural and artificial illumination produces light waves whose electric field vectors vibrate in all planes that are perpendicular with respect to the direction of propagation. If the electric field vectors are restricted to a single plane by filtration of the beam with specialized materials, then the light is referred to as plane or linearly polarized with respect to the direction of propagation, and all waves vibrating in a single plane are termed plane parallel or plane-polarized.
Introduction to Polarized Light - The human eye lacks the ability to distinguish between randomly oriented and polarized light, and plane-polarized light can only be detected through an intensity or color effect, for example, by reduced glare when wearing polarized sun glasses. In effect, humans cannot differentiate between the high contrast real images observed in a polarized light microscope and identical images of the same specimens captured digitally (or on film), and then projected onto a screen with light that is not polarized. The first clues to the existence of polarized light surfaced around 1669 when Erasmus Bartholin discovered that crystals of the mineral Iceland spar (more commonly referred to as calcite) produce a double image when objects are viewed through the crystals in transmitted light. During his experiments, Bartholin also observed a quite unusual phenomenon. When the calcite crystals are rotated about their axis, one of the images moves in a circle around the other, providing strong evidence that the crystals are somehow splitting the light into two different beams.
Polarized Light Microscopy -The polarized light microscope is designed to observe and photograph specimens that are visible primarily due to their optically anisotropic character. In order to accomplish this task, the microscope must be equipped with both a polarizer, positioned in the light path somewhere before the specimen, and an analyzer (a second polarizer), placed in the optical pathway between the objective rear aperture and the observation tubes or camera port. Image contrast arises from the interaction of plane-polarized light with a birefringent (or doubly-refracting) specimen to produce two individual wave components that are each polarized in mutually perpendicular planes. The velocities of these components are different and vary with the propagation direction through the specimen. After exiting the specimen, the light components become out of phase, but are recombined with constructive and destructive interference when they pass through the analyzer. Polarized light is a contrast-enhancing technique that improves the quality of the image obtained with birefringent materials when compared to other techniques such as darkfield and brightfield illumination, differential interference contrast, phase contrast, Hoffman modulation contrast, and fluorescence.
Max Berek (1886-1949) - Max Berek was a German physicist and mathematician, associated with the firm of E. Leitz, who designed a wide spectrum of optical instruments, in particular for polarized light microscopy and several innovative camera lenses. Professor Berek is credited as the inventor of the Leica camera lens system at their Wetzlar factory.
Sir David Brewster (1781-1868) - Sir David Brewster was a Scottish physicist who invented the kaleidoscope, made major improvements to the stereoscope, and discovered the polarization phenomenon of light reflected at specific angles. In his studies on polarized light, Brewster discovered that when light strikes a reflective surface at a certain angle (now known as Brewster's Angle), the light reflected from that surface is plane-polarized. He elucidated a simple relationship between the incident angle of the light beam and the refractive index of the reflecting material.
Shinya Inoué (1921-Present) - Shinya Inoué is a microscopist, cell biologist, and educator who has been described as the grandfather of modern light microscopy. The pioneering microscopist heavily influenced the study of cell dynamics during the 1980s through his developments in video-enhanced contrast microscopy (VEC), which is a modification of the traditional form of differential interference contrast (DIC) microscopy. Inoué also made significant contributions to the investigation of biological systems with polarized light microscopy. His seminal work, "Video Microscopy," was published in 1986, and a second revised and updated edition, co-authored with Kenneth Spring, followed in 1997. The book is a cornerstone of microscopical knowledge and is highly regarded throughout the scientific community.
Edwin Herbert Land (1909-1991) - The founder of the Polaroid Corporation, Edwin Herbert Land was an American inventor and researcher who dedicated his entire adult life to the study of polarized light, photography and color vision. Perhaps Land's most famous contribution to science, however, was his development of instant photography. The invention was inspired by his three-year old daughter when she asked him why she could not instantly see a picture he had just taken of her on vacation. The one-step dry photographic process took Land three years to perfect, but his success was phenomenal.
Henri Hureau de Sénarmont (1808-1862) - Sénarmont was a professor of mineralogy and director of studies at the École des Mines in Paris, especially distinguished for his research on polarization and his studies on the artificial formation of minerals. He also contributed to the Geological Survey of France by preparing geological maps and essays. Perhaps the most significant contribution made by de Sénarmont to optics was the polarized light retardation compensator bearing his name, which is still widely utilized today.
Interactive Java Tutorials
Brewster's Angle - Light that is reflected from the flat surface of a dielectric (or insulating) material is often partially polarized, with the electric vectors of the reflected light vibrating in a plane that is parallel to the surface of the material. Common examples of surfaces that reflect polarized light are undisturbed water, glass, sheet plastics, and highways. In these instances, light waves that have the electric field vectors parallel to the surface are reflected to a greater degree than those with different orientations. This tutorial demonstrates the polarization effect on light reflected at a specific angle (the Brewster angle) from a transparent medium.
Double Refraction (Birefringence) in Iceland Spar - The first clues to the existence of polarized light surfaced around 1669 when Erasmus Bartholin discovered that crystals of the mineral Iceland spar (more commonly referred to as calcite) produce a double image when objects are viewed through the crystals in transmitted light. This interactive tutorial simulates viewing of a ball-point pen and a line of text through a crystal of Iceland spar, producing a double image.
Polarized Light Waveforms - The ordinary and extraordinary light waves generated when a beam of light traverses a birefringent crystal have plane-polarized electric vectors that are mutually perpendicular to each other. In addition, due to differences in electronic interaction that each component experiences during its journey through the crystal, there is usually a phase shift that occurs between the two waves. This interactive tutorial explores the generation of linear, elliptical, and circularly polarized light by a pair of orthogonal light waves (as a function of the relative phase shift between the waves) when the electric field vectors are added together.
Polarization of Light - When light travels through a linear polarizing material, a selected vibration plane is passed by the polarizer, while electric field vectors vibrating in all other orientations are blocked. Linearly polarized light transmitted through a polarizer can be either passed or absorbed by a second polarizer, depending upon the electric vector transmission azimuth orientation of the second polarizing material. This tutorial explores the effect of rotating two polarizers on an incident beam of white light.
Polarization of Light (3-D Version) - When non-polarized white light encounters a linear polarizer that is oriented with the transmission azimuth positioned vertically to the incident beam, only those waves having vertical electric field vectors will pass through. Polarized light exiting the first polarizer can be subsequently blocked by a second polarizer if the transmission axis is oriented horizontally with respect to the electric field vector of the polarized light waves. The concept of using two polarizers oriented at right angles with respect to each other is commonly termed crossed polarization and is fundamental to the concept of polarized light microscopy. This tutorial explores the effects of two polarizers having adjustable transmission axes on an incident beam of white light, and enables the visitor to translate the optical train in three dimensions.
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.
Electromagnetic Wave Propagation - Electromagnetic waves can be generated by a variety of methods, such as a discharging spark or by an oscillating molecular dipole. Visible light is a commonly studied form of electromagnetic radiation, and exhibits oscillating electric and magnetic fields whose amplitudes and directions are represented by vectors that undulate in phase as sinusoidal waves in two mutually perpendicular (orthogonal) planes. This tutorial explores propagation of a virtual electromagnetic wave and considers the orientation of the magnetic and electric field vectors.
Polarized Light Virtual Microscopy Java Tutorials
Polarized Light Virtual Microscopes - When a birefringent material is placed between crossed polarizers in an optical microscope, light incident upon the material is split into two component beams whose amplitude and intensity vary depending upon the orientation angle between the polarizer and permitted vibration directions of the material. Use this link to explore our tutorials on polarized light microscopy.
Selected Literature and Web Resources
Polarized Light Literature References - A number of high-quality books and review articles on polarized light microscopy have been published by leading researchers in the field. This section contains periodical location information about these articles, as well as providing a listing of selected original research reports and books describing the classical techniques of optical crystallography and polarized light microscopy.
Polarized Light Microscopy Web Resources - Although much neglected and undervalued as an investigative tool, polarized light microscopy provides all the benefits of brightfield microscopy and yet offers a wealth of information, which is simply not available with any other optical microscopy technique. This section is a compendium of web resources focused on all aspects of polarized light microscopy, optical crystallography, and related techniques.
Douglas B. Murphy - Department of Cell Biology and Microscope Facility, Johns Hopkins University School of Medicine, 725 N. Wolfe Street, 107 WBSB, Baltimore, Maryland 21205.
Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657.
Matthew J. Parry-Hill, Robert T. Sutter, Thomas J. Fellers, 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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