Stokes, George Gabriel (1819–1903)

Irish physicist who studied the viscosity (resistance to relative motion) of fluids. This culminated in Stokes' law, F = 6πηrv, which applies to a force acting on a sphere falling through a liquid, where η is the liquid's viscosity and r and v are the radius and velocity of the sphere. He became a baronet in1889.

In 1852 Stokes gave the first explanation of the phenomenon of fluorescence, a term he coined. He noticed that ultraviolet light was being absorbed and then re-emitted as visible light.

This led him to use fluorescence as a method to study ultraviolet spectra.

Stokes was born at Skreen, in County Sligo, Ireland. He was educated at Bristol and Cambridge, where he was appointed Lucasian professor of mathematics in 1849.

Stokes's investigation into fluid dynamics in the late 1840s led him to consider the problem of the ether, the hypothetical medium for the propagation of light waves. He showed that the laws of optics held if the Earth pulled the ether with it in its motion through space, and from this he assumed the ether to be an elastic substance that flowed with the Earth.

Stokes realized in 1854 that the Sun's spectrum is made up of spectra of the elements it contains. He concluded that the dark Fraunhofer lines are the spectral lines of elements absorbing light in the Sun's outer layers.

Stokes's works include Mathematical and Physical Papers 1880–1905, On Light 1884–87, and Memoirs and Scientific Correspondence 1907.