Q.What is Huygens' principle?

A wavefront is the locus of all points that are in the same phase (state) of vibration at a given instant; because points on a wavefront are all in step, a wavefront is always perpendicular to the direction of the wave's propagation (the ray). A point source at a finite distance produces spherical wavefronts, a line (extended) source produces cylindrical wavefronts, and any source located effectively at infinity produces plane wavefronts. Huygens' principle is a geometrical construction rule stating that every point on a wavefront acts as a source of secondary spherical wavelets, spreading outward with the wave's own speed; the new wavefront at a later time $t$ is the common forward tangent (the 'envelope') of all these secondary wavelets, each having grown to radius $ct$ from its originating point. This construction correctly reproduces spherical wavefronts from a nearby point source and plane wavefronts from a distant source, and it explains how a wavefront propagates without needing to track individual rays. Its one acknowledged shortcoming is that it …