Power and application of a lens

Question 1 Find the power of concave lens of focal length 2m?

Question 2 Find the focal length of concave lens whose power is -2 D?

Question 3 The power of lens is 2.5 D.What is the focal length and the type of lens?

Question 4 A convex and concave lens of focal length 50 cm and 40 cm are combined together,what is the power of the combination?

Question 5 What is meant by power of a lens?

Question 6 What is the SI unit of power of lens?

Question 7 Give few applications of lens?

Question 8 Define Diopter?

Power of a lens

Power of a lens is the ability to converge or diverge the ray of light falling on it.

Power of a lens is defined as reciprocal of focal length of the lens.




A lens of small focal length has large power of converging or diverging a parallel beam of light.

Unit of power is Diopter (D)

Power of lens is 1 diopter if its focal length is 1 m.

Consider 2 lenses with focal length.When lens combine,they behave as a single lens of focal length.
  1         1         1

—- = —– + ——–

  F         f1          f2



P= —–


P1 = ——



P2 = —–


Application Of lens

1)Convex lens are used in camera,astronomical telescope,terrestrial telescope.

2)Convex lens of small focal length is used in microscope to study biological specimen.

3)It is used in reading glass.

4)It is used to correct hypermetropia defect.

5)Concave lens is used to correct myopia defect of human eye.

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