5.1 Reflection

The Law of Reflection

When light hits a smooth surface, the angle of incidence equals the angle of reflection, both measured from the normal (a line perpendicular to the surface). This holds for flat (plane) and curved mirrors alike. Rays, incident ray, reflected ray, and the normal all lie in the same plane.

Plane Mirrors and Image Formation

A plane mirror produces a virtual, upright, same-size image located as far behind the mirror as the object is in front. Because reflected rays only appear to come from behind the mirror, the image cannot be projected onto a screen — light rays diverge after reflecting.

Curved Mirrors: Concave vs. Convex

Concave mirrors curve inward and can converge rays to form real, inverted images (when object is beyond the focal point) or virtual, upright, magnified images (when object is inside focal point). Convex mirrors curve outward, always diverging rays to form virtual, upright, reduced images — used for wide-view mirrors.

Key terms

Law of Reflection

The angle of incidence equals the angle of reflection, both measured from the normal to the surface.

Normal (in optics)

An imaginary line perpendicular to a reflecting or refracting surface, used as the reference for measuring angles.

Plane mirror image

Virtual, upright, and the same size as the object; located as far behind the mirror as the object is in front.

Concave mirror

Curves inward; can form real, inverted images (object beyond focal point) or virtual, upright, magnified images (object inside focal point).

Convex mirror

Curves outward; always forms virtual, upright, reduced images by diverging reflected light.

Try an AP-style question

Light strikes a mirror at 30° from the normal. What is the angle of reflection?

1. 60°
2. 30°
3. 90°
4. 15°