A ray is incidenton a plain mirror. Its reflected ray is perpendiculartothe incident ray. Find the angle of incidence.

We know that angle of incidence is equal to angle of reflection.

Angle form between the incident and reflected ray= 90 degree as its reflected ray is perpendicular to the incident ray.

Let the angle of incidence be y and angle of reflection be also y {as angle of incidence = angle of reflection}

y+y= 90

or, 2y = 90

or, y = 90/2

or y = 45 degree

So angle of incidence 45 degree.