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5.5 Images formed by a plane mirror

Locating the image

As with curved mirrors and lenses, the location of an image in a plane mirror can be found by extending the reflected rays. The image is seen at the intersection of these rays. Plane mirrors give an image which is the same size and attitude or orientation as the object and has the same relative distance to the mirror.
In previous mathematical courses you learned a simple graphical method to locate the image formed by a plane mirror, based on two rules:

  • The object and image are located on a line perpendicular to the mirror relative to the object and
  • the distance from the mirror to the image is equal to the distance from the mirror to the object.

Although this technique is useful and effective, it is not a realistic model of how we see images. Binocular vision enables humans to locate an image using telemetry.

The characteristics of the image

  1. The image is located behind the mirror.
  2. The image is virtual.
  3. The image is the same size as the object.
  4. The image is upright, with the same attitude as the object.
  5. The image undergoes a lateral inversion in relation to the object.
  6. The image is situated at the same distance from the mirror as the object.

Object position

Image characteristics

(principle points)


Attitude (orientation)



at infinity



infinitely small

at infinity

between infinity the mirror



M = 1

di = do

Path of light
Mirrors in our lives