(a) Define a wavefront. Use Huygen' principle to shown diagrammatically the propagation of a plane wavefront from the intant `t_(1)=0` to later time `t_(2)`.
(b) State briefly two features which can distinguish the characteristic features of an interference pattern from those observed in the diffraction pattern due a single slit.

Explanation

(a) Wavefront : The locus of all the particles of the medium, which at any instant are vibrating in the same phase, is called the wavefront.
Huygen's priciple is a geometrical constrution which is used to determine the new position of a wavefront at a later time its given position at any instant.

Consider a point source of light. Let `AB` be a section of primary spherical wavefront at any time `t,` The new position of wavefront at time `t+Deltat,` conside a number of points `a,b,c,d-` on the primary wavefront. The partition of the wavelet, which lies just on the back of the secondary wavefront, `O` is equal to `180^(@)` adn likewise the factor `(1)/(2)(1+costheta)` is equal to zero. Therefore, the forward secondary wavefront `A'B'` is the new position of the wavefront at time `t+Deltat`.
(b) Difference between Interference and Diffraction.