Determination of the focal length of Convex lens by obtaining the image of a distant object


To determine the focal length of a convex lens by obtaining the image of a distant object.

Apparatus Required

  • Convex lens
  • A meter scale
  • A screen
  • A lens holder
  • A screen holder


For an object placed at infinity (practically a distant object), the image formed by a convex lens lies at the focus F of the convex lens. Distance between the optical centre O and the point where image is formed (i.e., focus F of the lens) is called focal length of the lens.

Image obtained is real and lies on other side of the lens. Moreover image is inverted and small. It can be easily obtained on a small screen.


  • Convex lens is mounted in the holder.
  • A distant object i.e., an electric pole was selected.
  • Screen mounted on screen holder was placed behind the lens.
  • Screen was moved towards or away from the lens to obtain a sharp image of the electric pole.
  • Distance between the pole of the mirror and screen was measured with the help of a metre scale.
  • Repeated the above procedure by selecting a distant tree and a distant telephone pole.
Fig:- Determination of focal length of a thin convex lens


Least count of metre scale = 1 mm

S. No. Distant ObjectDistance between lens and screen
(Focal length)
1.Polef1 = 24.8 cm
2.Tree 1f2 = 25 cm
3.Tree 2f3 = 25 cm


Mean focal length = \inline \frac{f_{1} + f_{2} + f_{3}}{3} = \inline \frac{24.8 + 25 + 25}{3} = 24.93 cm


The approximate value of focal length of the given convex lens = 24.93 cm.


  1. Lens and screen were held upright.
  2. Care was taken that sharp image of the distant object was obtained because the blurred image would given wrong result.
  3. Metre scale was correctly positioned before the eye level.
  4. Least count of the metre scale was correctly noted.
  5. No obstacle should be there in the path of rays of light from the distant object incident on the convex lens.
  6. The image of sun should never be seen directly with the naked eye and sunlight should never be focused with a convex lens on any part of the body or any inflammable materials, as it can cause damage.

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