Sign Convention, Mirror Formula and Magnification

To measure distances with respect to a curved mirror, sign convention is followed.

  1. All distances are measured from the pole of the mirror.
  2. The distances measured in the direction of incident light, are taken as positive.
  3. The distances measured in opposite direction of incident light, are taken as negative.
  4. The distances above the principal axis are taken positive, whereas those below it are taken as negative.

Mirror Formula

In concave mirror, when an object is placed at 2f (centre of curvature), the image is formed at 2f. If f be the focal length of the concave mirror, u distance of object and v the distance of image, then 

u = -2f

v = -2f

1/f = 1/-2f + 1/-2f

1/f = 1/v + 1/u

This is called mirror formula.

Magnification in Spherical Mirrors

A spherical mirror can produce magnified image of an object. The ratio of the size of the image to the size of the object is called linear magnification (M).

M = (size of image) / (size of object) = I/O = v/u

where v = image distance from mirror, u = object distance from mirror.