CBSE Class 12-science Answered

A ray of light is incident on the surface along with another ray which is parallel and also incident to this ray on this surface. 

AB is incident at an angle ‘ i ‘ on the reflecting surface MN. 

These rays are incident from the surface.

When we draw a perpendicular from point ‘A’ to this ray of light, A line joining the Point A and point B will be formed.

This is called as wavefront and it is incident on the surface.

As the incident wavefront is carrying two points, A and B, we can say that from point B to point C the light is travelling some distance.

Let ‘ v ‘ represent the speed of the wave in the medium and  ‘ r ‘ represent the time taken by the wavefront from the point B to C then the distance

BC = vr

To construct the reflected wavefront we draw a circle of radius vr from the point A.

Let CE represent the tangent plane drawn from the point C to this circle.

Thus,

AE = BC = vr

Considering the triangles EAC and BAC we observe that they are congruent.

As these triangles are congruent, measure of their corresponding angles 'i' and 'r' will also be equal.

This is law of reflection that states that angle of incidence 'i' is equal to angle of reflection which is 'r'