What reflection is

A mirror you look into is a plane mirror. When light falls on its surface and turns back, that is reflection.

After Figure 5.3 — a ray striking the mirror at right angles comes straight back along itself.

The words for a reflection

Figure to be added

Figure 5.4 — a plane mirror MN; ray AB strikes it at B and is reflected along BC; BX is the normal at B, drawn dashed; the angle ABX is marked i and the angle CBX is marked r.

	Name
AB	Incident ray — the ray falling on the mirror
BC	Reflected ray
BX	Normal — an imaginary line at right angles to the mirror at the point of incidence
ABX	Angle of incidence (i) — between the incident ray and the normal
CBX	Angle of reflection (r) — between the normal and the reflected ray

Measured from the normal, not the mirror

Both angles are measured between the ray and the normal. A common mistake is to measure from the mirror's surface instead, which gives 90° minus the right answer.