Wood, Robert Williams (1868-1955) was an American physicist who became known for his work in physical optics (the branch of physics concerned with the properties of light). His inventions and contributions were numerous.

Wood was born in Concord, Massachusetts. He graduated from Harvard University in 1891. He also studied at Chicago and Berlin. He became professor of experimental physics at Johns Hopkins University in 1901, where he worked for the rest of his life, retiring in 1938 as professor emeritus.

A principal interest of Wood's was in spectrum analysis of metallic vapor. Spectrum analysis allows scientists to determine the chemical composition and temperature of such bodies as an object heated in a laboratory. The atoms or molecules of all substances give off light when heated to high temperatures. The pattern of light given off is different for every substance. Thus, experts can determine the chemical composition of a substance by analyzing its spectrum. The results of his experiments in this area were of great significance to the advancement of atomic physics, particularly in terms of theory of the atom proposed by Niels Bohr.

Wood also did research in resonance radiation. Each kind of atom has resonance, a natural frequency of emitting light or radio waves. Wood also made a significant improvement to diffraction grating, a glass plate with lines ruled at small, equal intervals. Light can pass only between the lines, and the slits are about as far apart as a wavelength of light. If a parallel beam of white light strikes the grating, a pattern of light of various colors appears on a screen beyond the grating. The colors appear because white light consists of different colors. These colors have different wavelengths, and the longer wavelengths are diffracted at greater angles. Scientists can identify a substance by the pattern of colors it produces through a diffraction grating.

Wood also pioneered in the development of color photography. He was a member of the National Academy of Sciences and a fellow of the Royal Society.