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Cambridge IGCSE Physics · 0625

Chapter 3: Waves — Part 2

Topic 3.2.1 · Reflection of light

Law of reflection

The normal is a line at 90° to the surface at the point of incidence.

Angle of incidence i and angle of reflection r are measured from the normal, not from the mirror.

For reflection, i = r.

Ray diagram for reflection at a plane mirror: incident ray, normal, reflected ray, angle of incidence i equal to angle of reflection r, both measured from the normal.
Diagram 1: Ray diagram for reflection at a plane mirror: incident ray, normal, reflected ray, angle of incidence i equal to angle of reflection r, both measured from the normal.

Plane-mirror image

The image in a plane mirror is: same size, same distance behind the mirror as the object is in front, and virtual (rays do not actually meet; they only appear to when extrapolated backwards).

Use simple constructions: draw two rays from a point on the object, reflect with i = r, extend the reflected rays back behind the mirror to locate the image.

Plane-mirror image: object and virtual image equal distance behind the mirror, same size, laterally inverted, with two rays extrapolated behind the mirror.
Diagram 2: Plane-mirror image: object and virtual image equal distance behind the mirror, same size, laterally inverted, with two rays extrapolated behind the mirror.

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