Describe the formation of an optical image by a plane mirror and give its characteristics, i.e. same size, same distance from mirror, virtual

Published by Patrick Mutisya · 14 days ago

Cambridge IGCSE Physics 0625 – 3.2.1 Reflection of Light

3.2.1 Reflection of Light

Objective

Describe how an optical image is formed by a plane mirror and state its characteristics: the image is the same size as the object, appears the same distance behind the mirror as the object is in front of it, and is virtual.

Key Concepts

  • A plane mirror has a flat reflecting surface.

  • Incident rays strike the mirror and are reflected according to the law of reflection.

  • The reflected rays appear to diverge from a point behind the mirror – this point is the image.

Law of Reflection

The angle of incidence (\$i\$) is equal to the angle of reflection (\$r\$):

\$ i = r \$

Both angles are measured with respect to the normal (a line perpendicular to the surface at the point of incidence).

Formation of the Image

  1. Draw the object in front of the mirror.

  2. For at least two points on the object, draw incident rays striking the mirror.

  3. Apply the law of reflection to each incident ray to obtain the reflected ray.

  4. Extend the reflected rays backwards (behind the mirror) as dotted lines.

  5. The point where the extensions intersect is the location of the image point.

Suggested diagram: Ray diagram showing an object in front of a plane mirror, incident and reflected rays, and the virtual image formed behind the mirror.

Characteristics of the Image Formed by a Plane Mirror

laterally inverted

CharacteristicExplanation
Same size as the objectThe reflected rays preserve the angles between them, so the image is not magnified or reduced.
Same distance behind the mirror as the object is in frontIf the object is \$d\$ metres from the mirror, the image appears \$d\$ metres behind the mirror.
VirtualThe reflected rays diverge; they do not actually meet. The brain extrapolates them back to a point behind the mirror, so the image cannot be projected onto a screen.
The left‑right orientation of the object is reversed in the image (e.g., text appears backwards).

Summary

When light strikes a plane mirror, it is reflected such that the angle of incidence equals the angle of reflection. By extending the reflected rays behind the mirror, we locate a virtual image that is:

  • Upright and the same size as the object,

  • Located the same distance behind the mirror as the object is in front of it,

  • Virtual (cannot be captured on a screen) and laterally inverted.

Typical Exam Question

Question: An object is placed 0.30 m in front of a plane mirror. State the position, size and nature of the image formed.

Answer: The image is 0.30 m behind the mirror, has the same size as the object, and is virtual (and laterally inverted).