Huygens' Theory

Huygens' wave theory needs a concrete geometric RULE to actually predict how a wavefront's shape at one instant determines its shape a little later -- without such a rule, simply saying 'light is a wave' would explain nothing quantitatively. This section builds that rule in three steps: first identifying what kinds of sources produce light (primary vs. secondary), then defining precisely what a WAVEFRONT is, and finally stating Huygens' Principle itself -- the geometric construction, based on treating every point of a wavefront as a fresh miniature source, that lets the wavefront's future shape be constructed from its present shape. This machinery, once established, is what the next two sections use to derive the laws of refle …