Optics

While still a student, Brewster began his extensive optical research. Of his thirty papers in the Philosophical Transactions of the Royal Society, all involved optics. A tireless experimenter, Brewster investigated phenomena including polarization, double refraction, and metallic reflection. He also invented the kaleidoscope, established Brewster's law, and helped create the fields of optical mineralogy and photoelasticity. He was highly regarded by contemporaries, who called him the "Kepler of optics" and the "father of modern experimental optics."

Despite recognizing the usefulness and beauty of Thomas Young's law of interference, Brewster believed that the hypothesis regarding light as waves remained unproven. Yet, his rejection of the wave theory did not stem from scientific inability or incompetence. Rather, Brewster's religious, methodological and epistemological views conflicted with those of the wave theorists.

In particular, he objected to the hypothetical luminiferous ether, an undetectable medium filling all space so that light would have something in which to create waves. Choosing to describe optical effects in terms of particles and forces, Brewster nevertheless did not blindly follow Sir Isaac Newton's corpuscular theory. While making significant theoretical alterations, he passionately defended the Newtonian emission theory until the end of his life.

A prolific writer, Brewster published numerous popular books, encyclopedia articles, reviews, and papers. Totalling several hundred items, his publications encompass a wide variety of subjects including astronomy, mechanics, magnetism, geology, color blindness, photography, and the philosophy of science. He also wrote several historical works including the definitive nineteenth-century biography of Newton entitled Memoirs of the Life, Writings, and Discoveries of Sir Isaac Newton (1855).

Also a key figure in scientific organization, Brewster helped found the Edinburgh School of Arts (1821), the Royal Scottish Society of Arts (1821) and the British Association for the Advancement of Science (1831), the latter the major disseminating force of science in nineteenth-century Britain. Despite his irascible, sometimes combative personality, Brewster contributed much to physical optics in particular and Victorian science in general.

A Treatise on New Philosophical Instruments, for Various Purposes in the Arts and Sciences, 1813

A Treatise on the Kaleidoscope, 1819

The Life of Sir Isaac Newton, 1831

A Treatise on Optics, 1831

A Treatise on Magnetism, 1837

The Martyrs of Science: Lives of Galileo, Tycho Brahe, and Kepler, 1841

Memoirs of the Life, Writings, and Discoveries of Sir Isaac Newton, 1855

The Stereoscope: it History, Theory, and Construction, 1856

The History of the Invention of the Dioptric Lights and their Introduction into Great Britain, 1862

"Brewster, David." Edgar W. Morse. Dictionary of Scientific Biography, 2: 451-454

"Brewster, David." Robert Hunt. Dictionary of National Biography, 6: 299-303

‘Martyr of Science’: Sir David Brewster 1781-1868. edited by A. D. Morrison-Law and J. R. R. Christie. Edinburgh: Royal Scottish Museum Studies, 1984

"The Tenacity of Newtonian Optics in England: David Brewster, the Last Champion." Henry John Steffens, in The Development of Newtonian Optics in England 137-151. New York: Science History Publications, 1977

Brewster determined a simple relationship relating refraction, reflection and polarization.

Refraction occurs when light rays change direction due to a change in the medium they are travelling through. For example, light changes direction when entering water or glass. When light enters certain substances, however, such as calcite, it is doubly refracted emerging as two beams instead one.

If a plate of polarizing material, such as tourmaline, is rotated in front of these emerging beams one will be blocked while the other is not, depending on the plate's relative position. Analogously, a beam reflected off a glass plate causes the intensity of the reflected light to vary according to the angle of the plate. These examples illustrate the phenomena of polarization, by double refraction and reflection respectively.

When the incident light hits the material at a certain angle, the refracted or reflected light is fully polarized. Brewster observed this special polarizing angle for numerous substances including glass, water, and many minerals. In doing so, he discovered a simple law: The index of refraction of the substance equaled the tangent of the polarizing angle. This relationship, now called Brewster's law, means that when light falls on a substance at the polarizing angle, the refracted and reflected rays are at right angles to one another.

Although Brewster advocated the emission theory of light, his many experimental results were reinterpreted by others to fit within the undulatory (wave) theory of light.

Development of Concepts of Physics. A. B. Arons. Addison-Wesley, Massachusetts, 1965

Light: Principles and Experiments. George S. Monk. New York: Dover Publications, 1963 (Second edition)

The Rise of the Wave Theory of Light. Jed. Z Buchwald. Chicago: University of Chicago Press, 1989