#### Question

Explain the construction of plane wavefront using Huygens’ principle.

#### Solution

a. A plane wavefront is formed when point of observation is very far away from the primary source.

b. Let PQR represent a plane wavefront at any instant. According to Huygens’ principle,all the points on this wavefront will act as secondary sources of light sending out secondary wavelets in the forward direction.

c. Draw hemispheres with P, Q, R…. as centres and ‘ct’ as radius. The surface tangential

to all such hemispheres is P_{1}Q_{1}R_{1}…. at instant ‘t’. It is a new wavefront at time ‘t’.

d. The plane wavefronts is propagated as plane waves in homogeneous isotropic medium.They are parallel to each other.

e. PP1N1, QQ_{1}N_{2}, RR_{1}N_{3} are the wave normals at P, Q, R respectively. These wave normals show the direction of propagation of plane wavefront.

f. The new wavefront P_{1}Q_{1}R_{1} is parallel to primary wavefront PQR.