in optics, statement that light traveling between two points seeks a path such that the number of waves (the optical length between the points) is equal, in the first approximation, to that in neighbouring paths. Another way of stating this principle is that the path taken by a ray of light in traveling between two points requires either a minimum or a maximum time. Thus, two beams of light diverging from a distant object point and converged by a lens to an image point will have identical optical path lengths. Fermat's principle was first enunciated in 1658 by Pierre de Fermat (Fermat, Pierre de), a French mathematician. It is useful in the study of optical devices.