Sir David Brewster (born December 11, 1781,Jedburgh,Roxburghshire, Scotland—died February 10, 1868, Allerby,Melrose, Roxburghshire) was a Scottish physicist noted for his experimental work inoptics andpolarized light—i.e.,light in which all waves lie in the same plane. When light strikes a reflective surface at a certain angle (called thepolarizing angle), the reflected light becomes completely polarized. Brewster discovered a simple mathematical relationship between the polarizing angle and therefractive index of the reflective substance. This law is useful in determining the refractive index of materials that areopaque or available only in small samples.

Brewster was educated for the ministry at theUniversity of Edinburgh, but his interest inscience deflected him from pursuing this profession. In 1799 he began his investigations of light. His most important studies involvedpolarization, metallicreflection, and light absorption. He was elected a fellow of theRoyal Society in 1815, and he invented thekaleidoscope the following year. He was knighted in 1831. In the early 1840s he improved thestereoscope by utilizing lenses to combine the two dissimilar binocular pictures and produce the three-dimensional effect. Brewster was instrumental in persuading the British to adopt thelightweight, flatFresnel lens for use inlighthouses. In 1838 he became principal of the United College of St. Salvator and St. Leonard of theUniversity of St. Andrews and in 1859 became principal of the University of Edinburgh.

Of Brewster’s numerous published works, hisTreatise on Optics (1831) andMemoirs of the Life, Writings and Discoveries of Sir Isaac Newton (1855) are probably the most important.

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