‘F = ma’ optics

Fermat’s principle may be used to derive an equation, of the form F=m a, governing the shape of a light ray in a medium of varying refractive index. Many interesting problems in gradient?index optics that ordinarily require considerable computation may therefore be solved very simply by analogy to familiar mechanical problems. This approach also provides the means of thoroughly exploring the optical?mechanical analogy at a much more elementary level than is usual