Question

A Ray of light strikes the glass slab at an angle 50 degree with the surface of the glass slab what will be the angle of incidence​

Answer 1

The angle of incidence is measured between the ray of light and the normal at the point the ray strikes the glass surface.

If you say the ray of light strikes the glass at 50 deg measured between the ray and the plane surface of the glass, then the angle of incidence is 90–50 = 40 degrees,