The phenomenon of reflection: definition and laws

What is reflection?

When a ray of light strikes a surface (a mirror, still water, polished metal, etc.), some of the light is sent off in a new direction: this is reflection. The ray that arrives is called the incident ray; the one that bounces off is called the reflected ray.

The normal: a key reference

To describe reflection, we draw the normal: a line perpendicular to the surface at the point where the ray strikes it. Angles are always measured between a ray and the normal, never between the ray and the surface itself.

• Angle of incidence i: between the incident ray and the normal
• Angle of reflection r: between the reflected ray and the normal

The law of reflection

Two simple rules summarise the whole phenomenon:

1. The incident ray, the normal and the reflected ray lie in the same plane.
2. The angle of incidence is equal to the angle of reflection: i = r

Angle of incidence i	Angle of reflection r
20 degrees	20 degrees
45 degrees	45 degrees
60 degrees	60 degrees

Practical example

If you point a laser pointer at a mirror at an angle of 30 degrees to the normal, the reflected ray will also emerge at 30 degrees, on the other side of the normal.

Common pitfall

Do not confuse the angle measured relative to the surface with that measured relative to the normal: a beam grazing the surface makes a large angle with the normal, not a small one.