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08 Lens Formula, Magnification and Power of Lens


Lens Formula: The lens formulagives the relation between image distance, object distance and focal length. The lens formula is given by:

1v  1u= 1f



v = image distance
u = object distance
f = focal length

The lens formula is true for all situations with appropriate sign conventions.

8.2 Magnification Produced by Lenses

Similar to the magnification produced by mirrors, magnification produced by lenses is the ratio of height of image to height of object. It can be represented as:

Magnification = height of imageheight of object

or                =     m    =




m = magnification

h2 = height of image

h1 = height of object


The linear magnification produced by a lens is equal to the ratio of image distance to the object distance. That is:

Magnification = image distanceobject distancem= vu



m = magnification

v = image distance

u = object distance


Important notes on magnification:

(i)The positive sign shows that the image is erect and virtual

(ii)The negative sign shows that the image is real and inverted


For a spherical lens, remember for the linear magnification (m):

Virtual positive, real negative.


  • The power of a lens is a measure of the degree of convergence or divergence of light rays falling on it.
  • The power of a lens is defined as the reciprocal of its focal length. It is represented as:
    Power of Lens = 1focal length of lens(in meters)


    P = 1f

    Where P = Power of the lens
    And f = focal length of the lens (in metres)

  • The unit of the power of a lens is dioptre. It is denoted by ‘D’.
  • A concave lens has a negative focal length, so the power of a concave lens is negative.
  • Power of convex lens is always positive

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