Question

theory of light strikes the glass slab at an angle of 50° with the surface of the slab what will be angle of incidence

Answer 1

he angle is incidence is that angle that the ray of light makes with the normal to the surface at the point on the glass slab where the ray of light is incident.

If the angle is measured with respect to the normal then the measured angle is the angle of incidence.

If the angle is measured with respect to the surface of the of the glass slab then the angle of incidence is the complement of the measured angle.