Cambridge O Level Physics · Syllabus 5054 · Waves
Law of reflection
What is Law of reflection?
The normal is the line drawn at right angles to a reflecting surface at the point where a ray strikes it; the angle of incidence is the angle between the incident ray and the normal, and the angle of reflection is the angle between the reflected ray and the normal. The law of reflection states that the angle of incidence equals the angle of reflection, and that the incident ray, the reflected ray and the normal all lie in the same plane.
This definition is part of the Waves chapter in Cambridge O Level Physics.
Questions students ask about Law of reflection
Why are angles in reflection and refraction always measured from the normal, not the surface?
Every optical law in this chapter — the law of reflection, \(n = \sin i/\sin r\), and \(n = 1/\sin c\) — is defined with respect to the normal, the line drawn at 90° to the surface at the point where the ray strikes it. Measuring from the surface instead gives the complement of the angle, \(90^\circ - i\), which is not what the law describes. Always draw the dashed normal first.