Reflection and image in a plane mirror

The laws of reflection

A plane mirror reflects light according to two simple laws:

1. The incident ray, the reflected ray and the normal (perpendicular to the mirror at the point of incidence) lie in the same plane.
2. The angle of incidence i is equal to the angle of reflection r, measured relative to the normal: i = r.

Constructing the image of a point

The image A’ of an object point A produced by a plane mirror is the mirror image of A with respect to the plane of the mirror. To construct it, we draw two rays from A, apply the law i = r to each, then extend the reflected rays behind the mirror: their (imaginary) point of intersection gives A'.

Characteristics of the image

• The image is always virtual (the reflected rays never actually intersect; they merely appear to originate from A').
• It is right-side up (not inverted) and the same size as the object.
• The object-to-mirror distance is equal to the mirror-to-image distance.

Practical example

If you stand 2 m from a plane mirror, your virtual image appears to form 2 m behind the mirror, i.e. 4 m away from you. This is why a mirror never ‘magnifies’ or ‘minimises’ an image.

Common misconception

Many pupils believe that a plane mirror reverses top and bottom like a lens: this is incorrect; it only reverses front and back (the left-right symmetry effect perceived when facing the mirror), not top and bottom.