Angle in, same angle out

The idea

The angle of incidence (i) is equal to the angle of reflection (r); a ray falling along the normal has both angles zero.

Change the angle

A ray falls on a mirror with an angle of incidence of 30°.

What do you expect the angle of reflection to be?

• 30°
• 60°
• It can be any angle

Measure and record

Shine the torch at each angle of incidence. Read both angles from the normal. The table fills in as you go.

A law of reflection

The angle of incidence (i) is equal to the angle of reflection (r). A ray falling along the normal has both angles zero.

This is a law of reflection. It is true at every angle you try.

• Angle of incidence (i) | Angle of reflection (r)
• 0° | 0°
• 20° | 20°
• 30° | 30°
• 45° | 45°
• 60° | 60°

Notes

The angle of incidence (i) is equal to the angle of reflection (r). A ray falling along the normal has both angles zero.

Check yourself

A ray falls on a plane mirror with an angle of incidence of 40°. What is the angle of reflection?

Answer: 40 °

i = r, so the angle of reflection is also 40°.

A ray falls on a mirror along the normal. What are the angles of incidence and reflection?

The angle between the incident ray and the reflected ray is 70°. What is the angle of incidence?

Answer: 35 °

i + r = 70° and i = r, so i = 70° ÷ 2 = 35°.

Is the angle of incidence equal to the angle of reflection at every angle?

• Both are 0° — correct. Yes. The ray goes straight back along the normal.
• Both are 90°. 90° would be a ray lying along the mirror. Along the normal, both angles are zero.
• i is 0° and r is 90°. The two angles are always equal. Here both are 0°.
• Yes, at every angle — correct. Yes. That is why it is a law of reflection.
• Only at 45°. It worked at 0°, 20°, 30°, 45° and 60°: at every angle you tried.
• Only for small angles. It worked at 60° too. It holds at every angle.