The law of reflection
A torch shines at a small mirror. Where does the beam go? A thin beam of light is called a ray. In the steps, square to means at 90° to.
- A ray comes down and hits the mirror.
- The normal is drawn square to the surface.
- It leaves at an angle, on the other side.
- Both angles, measured from the normal, are equal.
The rule
The beam leaves at the same angle it arrived at.
The ray that arrives is the incident ray. The ray that leaves is the reflected ray. The normal is a line at 90° to the mirror. The two rays and the normal also lie in one flat plane.
The angle of incidence equals the angle of reflection. Both are measured from the normal. The normal is at 90° to the mirror.
Write i for the angle of incidence. Write r for the angle of reflection.
Example: i = 35°, so r = 35°. The normal is 90° from the mirror. So the reflected ray is 90° − 35° = 55° from the mirror.
A ray along the normal has i = 0° and r = 0°. So it goes straight back.
Pick an angle. Read the angle of reflection.
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