Reflection of Light Waves

Abstract

What is light? In Chap. I of his Treatise on Light, Huygens argued that light consists of successive compressions and rarefactions of a subtle and all-pervasive medium—the æther—which are generated by vibrating particles inside of luminous bodies. This mechanism, he claimed, provides a plausible explanation for the singularly high speed of light, something the particle theory of light could not do. Huygens also attempted to demonstrate that his wave theory can account for the rectilinear propagation of light rays—despite the fact that a wave tends to spread spherically around its source. This he did by invoking what is now known as Huygens’ Principle. Huygens’ principle provides a very powerful recipe for predicting the advance of a wavefront, especially through inhomogeneous media; it is employed extensively nowadays in the fields of optics, acoustics, and even seismology. In what follows, Huygens attempts to use this technique for a more modest aim: to provide a physical explanation of the well-known law of reflection of light. His proof proceeds purely geometrically, assuming only Huygens’ principle.

It is evident that one could not demonstrate the equality of the angles of incidence and reflexion by similitude to that which happens to a ball thrown against a wall, of which writers have always made use.

—Christiaan Huygens