The Law of Reflection

Light behaves like a ball bouncing off a floor, but the angles are measured from the normal, not from the surface itself. Draw a dashed line perpendicular to the mirror right where the ray hits. The incoming ray makes some angle with that normal, called the angle of incidence. The reflected ray leaves on the other side of the normal at the exact same angle, called the angle of reflection. Both angles stay in the same plane as the normal, so the ray does not wander off to the side.

Measuring from the normal instead of the surface keeps the rule simple and consistent no matter how the mirror is tilted or curved at that point. If you tried to measure from the surface instead, a flat mirror and a curved mirror would need different setups. The normal always points straight out from the surface, so the same law of incidence equals reflection applies everywhere, from flat mirrors to tiny patches of a curved lens or mirror.

This single rule is the starting point for every ray diagram in the unit. Once you can find one reflected angle, you can trace a ray through multiple bounces, off two mirrors meeting at a corner, or into the curved mirrors used to form images later in the unit. The same idea, rays traveling in straight lines and obeying a strict angle rule, carries over when light bends into glass, except there the rule involves a change in speed instead of a simple mirror flip.