Question

What will be the nature of the wavefront of direct sunlight and why will it be so?

Answer 1

Hint: A wavefront is defined as a continuous locus of all those points in a medium which oscillate in the same phase. The physical view of a wavefront is the ripples on the water surface. When a stone is dropped in a water pond, the disturbance travels radially outwards and what we see are circular wavefronts travelling outwards.
To answer this question, we have to understand the nature of the wavefront which is transmitted from the Sun. Then we have to observe the wavefront which we on Earth are receiving.

Complete answer:
Wavefronts are basically of three types:
A.Spherical wavefronts: Spherical wavefronts refer to those which radiate out from a point source in the form of a circular ring with increasing diameter.
B.Cylindrical wavefront: These wavefronts refer to those which come out from a line source and radiate in the form of rays.
C.Planar wavefront: Planar wavefronts refer to those which originate from a plane traveling in straight lines.

The wavefront from the Sun is actually a spherical wavefront as it originates from a point source. But as the Earth is very small in comparison to the Sun and the distance between the Sun and the Earth is very huge. The spherical wavefronts originating from the Sun appear to be planar wavefront while it reaches the Earth as it seems to be like a straight line. The figure shown below will give a more clear idea about the reason.

Note: It must be noted that all the points on a wavefront serve as a point source of secondary spherical wavefronts. After a certain amount of time, the new position of the wavefront will be that of a surface tangent to these secondary wavelets.