Reflection from a smooth surface
In this interactive animation, you can rotate a laser beam and change the angle at which it hits a plane mirror. Move the laser around and observe what happens: as you change the direction of the incoming (or incident) beam, the direction of the reflected beam changes accordingly. We can express this quantitatively by saying: "The angle of incidence is equal to the angle of reflection". This is called the law of reflection. Let's take a moment to think about exactly what angles we are talking about: As it is shown in the animation, the angle of incidence is the angle between the incoming laser beam and the "normal" to the mirror surface. By a "normal", mathematicians mean a line that is perpendicular, or at right angles, to another line or surface. So the normal to the mirror surface is the line that is perpendicular to the mirror surface. This is the line relative to which we measure the angles of incidence and of reflection. This is a very important point for all of the following discussions about reflection and refraction. Obviously, the angle that the incoming laser beam makes with the mirror surface itself and the angle that the reflected beam makes with the mirror surface are also equal. However, these angles are not what we need to consider, and they have no special names.