The three laws of reflection

Any mirror obeys the three laws of reflection, flat, curved, convex or concave.

The three laws of reflection are

1. The angle between the incident ray and the normal is equal to the angle between the reflected ray and the normal

2. The incident ray, the normal and the reflected ray are all in the same plane

3. Incident ray and refracted ray are on different sides of the normal

Law 1 explained

The angle between the incident ray and the normal is equal to the angle between the reflected ray and the normal.

Diagram of incident ray normal line and reflected ray on a mirror

This means that `thetai` equals `thetar`

where

`thetai=` angle of incidence

`thetar=` angle of reflection

As the angle of incidence (`thetai`) increases, the angle of reflection (`thetar`) also increases and they are always equal to each other.

$Ray diagram showing refracted ray as angle of incidence increases$

$Ray diagram showing refracted ray as angle of incidence decreases$

Law 2 explained

The incident ray, the normal and the reflected ray are all in the same plane.

$Plane of incident ray, normal line and refracted ray$

`thetai=thetar`

Changing the direction of the incident ray changes the angle of the plane.

$Plane of incident ray, normal line and refracted ray changed$

`thetai=thetar`

Again the incident ray, the normal line and the reflected ray all lie in the same plane.

Law 3 explained

The incident ray and the reflected ray are on different sides of the normal.

$The incident ray and refracted ray must be on opposite sides of the normal line.$

The above diagram is wrong. The incident ray and the reflected ray can not be on the same side of the normal.

$Incident ray and refracted ray on opposite sides of the normal line$

The incident ray and reflected ray must be on different sides of the normal.