10.3 The Laws of Reflection

Why does a mirror bounce light back so predictably? Because reflection obeys two simple, exact laws. To state them, we first need a tidy way to talk about light.

Rays, Normal, and the Two Angles We draw light as straight lines with arrows, called rays (light travels in straight lines).

  • The ray striking the mirror is the incident ray; the one bouncing back is the reflected ray.
  • At the point where the ray hits, draw a line perpendicular (90°) to the mirror — the normal.
  • The angle between the incident ray and the normal is the angle of incidence (i); between the reflected ray and the normal, the angle of reflection (r).

(Note: the angles are measured from the normal, not from the mirror surface.)

Activity 10.4 — Measure i and r Shine a thin beam at a plane mirror, draw the incident ray, normal and reflected ray, and measure both angles with a protractor. Repeat for several different angles. Each time, the two angles come out equal. (And if the beam comes straight along the normal, both angles are 0° — it bounces straight back.)

Law 1: The Angles Are Equal The angle of incidence equals the angle of reflection: \;i = r.

Drag the slider to change the angle of incidence and watch the reflected ray mirror it exactly.

The law i = r
Change the angle of incidence — the reflected ray always matches it.
normal incident reflected

Activity 10.5 — One flat plane Catch the reflected beam on a paper sheet that sticks out past the table edge, then bend the sheet down along the edge. The reflected beam vanishes — and reappears when you flatten the paper. This shows the reflected ray lies in the same flat plane as the incident ray; bending the paper into a new plane breaks the alignment.

Law 2: All in One Plane The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane. Together with i = r, these are the two laws of reflection — and they hold for every mirror: plane, concave and convex.