Identify the angle of incidence.

The angle of incidence is the angle between the incident ray and the normal (the dashed line perpendicular to the mirror).
From the diagram, the angle of incidence is $40^\circ$.

The angle of reflection is the angle between the reflected ray and the normal.
From the diagram, the angle of reflection is $40^\circ$.

The diagram illustrates the Law of Reflection, which states that the angle of incidence is equal to the angle of reflection.

In this case, the angle of incidence ($40^\circ$) is equal to the angle of reflection ($40^\circ$).

The angle of incidence is $\boxed{40^\circ}$.