Bouncing by the rule
You tilt a mirror to send a torch beam round a corner. Which way does the beam go?
Step 1 of 3
- A ray hits the mirror. The normal is drawn square to the surface.
- The incident ray arrives at 40° from the normal.
- The reflected ray leaves at 40° too. The two angles are equal.
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The rule
The reflected ray makes the same angle as the incident ray. The normal is a line square to the surface. It is drawn at the point where the ray lands. The incident ray is the ray that arrives. The reflected ray is the ray that leaves. The angle of incidence is i. It is measured from the normal to the incident ray. The angle of reflection is r. It is measured from the normal to the reflected ray. A plane is a flat surface. So a plane mirror is a flat mirror.
| Law | What it says |
|---|---|
| Equal angles | The angle of reflection equals the angle of incidence. Both are measured from the normal. |
| One plane | The incident ray, the reflected ray and the normal lie in one plane. |
Worked example
A ray meets a plane mirror at 30° to its surface.
- The normal is at 90° to the surface. The angle from the normal is then 90° − 30° = 60°. So i = 60°.
- The angle of reflection is r = i = 60°.
- The angle between the two rays is 60° + 60° = 120°.
A ray that travels along the normal has i = 0°. It comes straight back along its own path.