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Lesson 4 of 8 · Textbook 5.2

The law of reflection

Six labels and one rule. Get the labels right and the rule is almost trivial — get them wrong and every angle you calculate will be wrong too.

The six things you must be able to label

TermDefinition
Incident rayThe light ray hitting the reflecting surface.
Point of incidenceThe point where the incident ray meets the surface.
NormalThe line perpendicular to the surface at the point of incidence. Always drawn dashed.
Angle of incidence, iThe angle between the incident ray and the normal.
Angle of reflection, rThe angle between the reflected ray and the normal.
Reflected rayThe light ray travelling away from the surface after bouncing off it.
The law The angle of reflection is equal to the angle of incidence.  i = r

Drag the incident ray

smooth, flat surface normal i r point of incidence
Angle of incidence i—
Angle of reflection r—

The red ray is the incident ray, arriving at the surface. The blue ray is the reflected ray, leaving it.
The mistake that loses the most marks Both angles are measured from the normal, never from the surface. If an exam diagram shows a ray at 30° to the surface, the angle of incidence is 90 − 30 = 60°, and the reflected ray leaves at 60° from the normal too.

Whenever a question gives you an angle, check which line it is measured from before you use it.

Science at work — lighting technical director In films, a lighting technical director studies how light is reflected off different surfaces in the real world, then reproduces objects as computer-generated imagery. Shadows add depth to characters and lighting enhances a scene, so the characters come to life.

Check yourself