Christian Huygens (1629-1695) was a Dutch Physicist who proposed a wave theory of light long before Maxwell's incredible discoveries. In 1678, while living in Paris (indeed, many Frenchmen seem to have studied the nature of light quite extensively), Huygens published his Trait' de la lumiere in which he described this wave theory, and the principle named after him comes from this text.
every point on a propagating wavefront serves as the source of spherical secondary wavelets, such that the wavefront at some later time is the envelope of these wavelets. If the propagating wave has a frequency, f, and is transmitted through the medium at a speed, v, then the secondary wavelets will have the same frequency and speed.
This principle is quite useful, for from it can be derived the laws of reflection and refraction [the latter often referred to as Snell's Law].
For a good graphical representation, R. A. Serway's Physics for Scientists and Engineers, 5th ed has a chapter on Geometrical Optics which has some nice pictures, as well as E. Hecth's Optics, a more advanced level book, again displaying Huygens's Principle graphically.
Philip Johnson, Physics Undergrad, Memorial University of Newfoundland
'Science is facts; just as houses are made of stones, so is science made of facts; but a pile of stones is not a house and a collection of facts is not necessarily science.'