Light rays follow a rather predictable pattern when it comes to reflection off a plane mirror surface. The angle at which the light ray approaches the mirror surface is equal to the angle at which it departs from the mirror. This is known as the law of reflection. In physics, the angles of approach are measured with respect to the normal line to the surface. The normal line is the imaginary line that is perpendicular to the mirror at the point that the light ray strikes the mirror. The angle between the normal line and the approaching or incident ray is known as the angle of incidence. Similarly, the angle between the reflected ray and the same normal line is known as the angle of reflection. According to the law of reflection, the angle of incidence is equal to the angle of reflection. A more detailed and exhaustive discussion of the law of reflection and associated terms can be found at The Physics Classroom Tutorial.