On the basis of Huygen.s wave theory, show that when light is incident on a plane mirror obliquely, angle of reflection is equal to angle of incidence.

Explanation

Huygen.s principle states that
(i) Every particle on the wavefront acts as a new source of waves called secondary wavelets.
(ii) These secondary wavelets travel in all directions with the speed of light.
(iii) The envelope of these wavelets in the forward direction at any instant gives the new wavefront.
Reflection of light : According to Huygen.s principle, each point on wavefront acts as a new source of waves. So wave takes time t seconds to move from wavefront AB to BA..

i.e., `AB.=BA.=vt`
where v is the velocity of wave
Now in `DeltaA A.B. and DeltaABA`
`angleABA.=angleAB.A.=90^(@)`
`AB.=BA.=vt`
`A A.=A.A` = common
Thus, the `Deltas` are congruent
`therefore angleBA A.=angleB.A.A`
`anglei=angler`