Q.State Huygens' principle. What is meant by a wavefront, and how does its shape differ for a point source close to the observer compared with a point source very far away?

A wavefront is defined as the locus of all points in a medium that are, at a given instant of time, vibrating in the same phase. Every point on a wavefront is at the identical stage of its oscillation as every other point on that same wavefront.

Huygens' principle consists of two parts taken together: first, every point on an existing wavefront acts as a source of secondary wavelets that spread out in all directions with the speed the wave has in that medium; second, the new wavefront at a later instant is the forward envelope, i.e. the common tangent surface, drawn to all of these secondary wavelets.

Because a point source radiates equally in all directions, the wavefront close to it is a sphere centred on the source, growing in radius as time passes. Far from the source, only a small patch of this very large sphere is considered, and over that small patch the curvature is negligible, so the wavefront is treated as a plane wavefront. The same plane-wavefront approximation also holds immediately after light from a point source has been rendered parallel by a converging lens (or a concave mirror), even though the source itself may be close by.

Recall the two-part statement of Huygens' principle and apply the geometry of a growing sphere at small and large distances from a point source.

• Confusing a wavefront (a surface of constant phase) with a ray (a line perpendicular to the wavefront, showing the direction of energy flow) -- they are different objects.
• Forgetting that 'plane wavefront' is always an approximation (valid over a small enough patch, or at a large enough distance), not a literal statement that the whole wavefront is flat.