Reflection

2.4 Plane mirrors

Describe the behaviour of light reflected by a plane mirror, after conducting experiments.

Predict the behaviour of the light

To predict the behaviour of light striking a mirror, we must first reproduce the phenomenon in a way that allows us to take measurements. The diagram below represents a small laser which we are using to strike three targets. The paths shown are the only ones by which light may leave a laser and strike a target. There is no other possible way to aim the laser in order to hit the targets.

It is possible to deduce a law only if the behaviour of the light can be reproduced and is consistent. Two things become immediately evident:

• The pencils of light that are projected in the plane of the sheet are reflected back along the same plane. Therefore, the plane of reflection of the light depends on the orientation of the mirror.
• In addition, we see that the angles made by the incident rays and the mirror are the same as those made by the reflected rays and the mirror.

THE ANGLE OF INCIDENCE = THE ANGLE OF REFLECTION

Predict the orientation of a mirror
Summary of the laws of reflection
Heron of Alexandria