Ware front: The continuous locus of all the particles vibrating in the same phase at any instant is called the wavefront.
*Laws of refection by Hygen Principle:-**
Let us consider a plane wave front AB incident on the place surface xy, incident rays are normal to the wavefront AB. Let in time t, the secondary wavelets reaches B' covering distance ct. Similarly, from each point on primary wavefront AB, secondary wivelets starts growing with the speed c. To find-reflected wave front after time t. Let us draw a sphere of radius ct taking B as center and now a tangent is drawn from B' on the sphere. The tangent B'A' represents reflected wavefront after time t.
For every point on wavefront AB a corresponding point lie on the reflected wavefront A'B'. So comparing two tringle DBAB' & DB'A'B we get AB' = A'B = ct, BB' = common
∠A=∠A'=90
Thus two triangles are congruent, hence ∠i=∠r
This proves the first law of reflection. Also, incident rays reflected rays and normal to them, all lie in the same plane. This gives the second raw of reflection.
