Huygens' Principle

In many cases, light waves are very much like water waves. One distinct difference, however, is that water waves are waves on a 2 dimensional plane (surface of the water) while light waves are waves within 3 dimensional space.

The wave theory, proposed by the Dutch physicist Christiaan Huygens, viewed light as an impulse moving in all directions. Consider a point P in space. If an impulse starts at P, then the effect of the impulse, after some time, will be equidistant from P in all directions -- one can visualize this impulse as an expanding sphere with center P.

Huygens called this sphere a front. Most importantly, every point on a front can be a source of new wavelets (act just like point P), and the envelope around those wavelets forms another front. In other words, a second front can be created from the first by making each point of the first front the origin of another impulse. All these impulses combine and appear as if the original front is expanding.

This model of light propagation through space by formation of spherical fronts is called Huygens' Principle.

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