normal

The normal is an imaginary line drawn perpendicular (at 90°) to a reflecting surface, right at the point where a light ray strikes it.

• The normal isn't a physical object you can see in real life — it's a reference line drawn purely to measure angles consistently, the same way a protractor's baseline helps measure any angle.
• Both key angles in reflection — the angle of incidence and the angle of reflection — are measured from the normal, never from the mirror's surface itself.
• When a light ray travels exactly along the normal (straight into the mirror), it reflects straight back the way it came, making both the angle of incidence and the angle of reflection zero.
• The normal matters even more for curved mirrors: at every point on a spherical mirror's surface, the normal points in a slightly different direction, which is exactly why parallel rays end up converging or diverging instead of staying parallel after reflecting.