What is Angle of Incidence?

In short

The angle of incidence is the angle between an incoming ray of light and the normal to the surface at the point where the ray arrives.

In simple words

Measure the angle from the normal, the line at right angles to the surface, and not from the surface. A ray that travels along the normal has an angle of incidence of $0^\circ$ and passes straight on or straight back. The angle of incidence decides the angle of reflection, and, when light enters another medium, the angle of refraction as well.

Example

A ray lies $20^\circ$ from a mirror's surface. The normal is $90^\circ$ from the surface, so the angle of incidence is $90^\circ - 20^\circ = 70^\circ$ . Many students wrongly say $20^\circ$ .