Laws of Reflection

Key points

• First law: angle of incidence = angle of reflection (i = r).
• Angles are measured from the normal, not from the surface.
• Second law: incident ray, normal and reflected ray lie in the same plane.
• Along the normal (i = 0) the ray returns on itself.

The two laws

i = r (both from the normal)

Solved examples

• A ray makes 30° with the mirror: i = 90° − 30° = 60°, so r = 60°.
• The angle between incident and reflected rays is 90°: i = r = 45°.
• Light falls along the normal: i = r = 0°.

Try it yourself

Goal: Discover that r = i (measured from the normal) by aiming a laser at a target.

• Move the slider to change the angle of incidence.
• Watch the angle of reflection and the angle from the surface.
• Hit the target, then press New target and try again.

Open the simulation full screen

Quick quiz

5 questions. Choose an answer for each, then submit to see your score and the explanations.