Final answer:
The angle of reflection will be 40 degrees, the same as the angle of incidence, according to the law of reflection.
Step-by-step explanation:
When light is incident upon a planar mirror, the angle of incidence is equal to the angle of reflection. This is known as the law of reflection. For the given scenario where light is incident with an angle of incidence of 40 degrees, the angle of reflection will also be 40 degrees.
The angle of incidence and the angle of reflection are always measured with respect to the normal (an imaginary line perpendicular) to the surface of the mirror.