MicroscopyPioneers
Pioneers in Optics: Sir David Brewster Michael W. Davidson National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32306
davidson@magnet.fsu.edu
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. Brewster was born in Jedburgh, Scotland, in 1781 and grew to become a brilliant student who entered the University of Edinburgh at the age of 12 to study the ministry. He was a prolific writer and became editor of the Edinburgh Magazine in 1802 and the Edinburgh Encyclopedia in 1808. In 1799 as a teenager, Brewster’s interests turned to physics with a keen focus on optics, and he constructed several telescopes while dab- bling in the physics of light. Brewster was a licensed minister of the Church of Scotland but never practiced this career, instead pursuing the finer aspects of optics and light. Brewster’s career blossomed while he was in his late
twenties and early thirties. At that time, he was intensely pursuing details of the theory of light and wrote his first paper Some Properties of Light in 1813. One of Brewster’s most important contributions to the science of physics was his work on polarization of light by reflection and with biaxial crystals. To aid in his experiments, Brewster oſten constructed his own tools and even improved many technical instruments of the period. 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. When the angle between the incident beam and the refracted beam equals 90 degrees, the reflected light becomes polarized. Tis rule is oſten used to determine the refractive index of materials that are opaque or available only in small quantities. Brewster was elected to the Royal Society in 1815 and
eventually was one of only a handful of scientists to be awarded all three principal medals of the society. For his work in optics, Brewster was awarded the Copley Medal in 1815, the Rumford Medal in 1818, and the Royal Medal in 1830. He also was a
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founder of the British Association for the Advancement of Science. An energetic enthusiast of color, Brewster invented the
kaleidoscope in 1816 and patented it the following year. He published his extensive studies on the theory, design, and construction of kaleidoscopes in 1819 in a volume entitled Treatise on the Kaleidoscope. Apparently there were some problems with the registration of his patent, because he was not able to enforce infringements and many companies began to offer custom versions of the kaleidoscope without paying royalties. Te instrument ignited a craze in the early nineteenth century and quickly became a household toy for both children and adults alike. Brewster was deeply interested in photography and had
many conversations with Fox Talbot about the design of Talbot’s Calotype process. He favored this process over the Daguerreotype and said, “While a Daguerreotype picture is much more sharp and accurate in its details than a Calotype, the latter possesses the advantage of giving a greater breadth and massiveness to its landscapes and portraits.” Brewster wrote hundreds of papers on optics and also
designed a famous variation of the stereoscope—the Brewster Stereoscope. He studied the theory of this instrument and improved the performance by adding refractive lenses to his model. He wrote what many consider the definitive treatise on the stereoscope, Te Stereoscope: Its History, Teory, and Construction. He also wrote his famous Treatise on Optics in 1831, and Memoirs of the Life, Writings, and Discoveries of Sir Isaac Newton in 1855. He became Sir David Brewster in 1831 when he was
knighted, and in the early 1840s he was a major proponent in the use of Fresnel lenses in lighthouses. In 1838 Brewster became principal of the United College of Saint Salvator and Saint Leonard of the University of Saint Andrews, and in 1859 he became principal of the University of Edinburgh.
Brewster’s Angle and Polarized Light When considering the incidence of non-polarized light
on a flat insulating surface, there is a unique angle at which the reflected light waves are all polarized into a single plane. Tis angle is commonly referred to as Brewster’s angle and can be easily calculated using the following equation for a beam of light traveling through air:
n = sin(θi)/sin(θr) = sin(θi)/sin(θ90-i) = tan(θi)
where n is the refractive index of the medium from which the light is reflected, θ(i) is the angle of incidence, and θ(r) is the angle of refraction. By examining the equation, it becomes
doi:10.1017/S1551929511001246
www.microscopy-today.com • 2011 November
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