To find focal length of a Concave Mirror

Objective -

To determine the focal length of a concave mirror, by obtaining image of a distant object.


  1. A concave mirror, like a plane mirror, obeys the laws of reflection of light.
  2. Rays of light from object - The rays of light coming from a distant object e.g. sun or a distant building can be considered to be parallel to each other. When these parallel rays of light fall on a concave mirror along its axis, reflect and meet at a point in front of the mirror, which is called as Principal focus of the mirror.
  3. realinverted and very small image size is formed at the focus of the mirror.
  4. Focal Length - The distance between the pole P of the concave mirror and the focus F is the focal length of the concave mirror. Thus, the focal length of a concave mirror can be estimated by obtaining a 'Real image' of a distant object at its focus, as shown in the figure.