Conduct an experiment to find out the focal length of the lens.

Aim: To find the focal length of the lens.

Requirements: Bi Convex Lens, Metre Scale, V Stand, Screen, an Object(i.e., Candle)


  1. Take a meter scale and keep it on a long table,
  2. Place the V-stand on the table along the meter scale.
  3. Keep the Convex lens on the V- stand along with the meter scale.
  4. Keep the object (i.e., Candle) in front of the lens and measure its distance from the lens and note it down on the table. (Say it is 5cm)
  5. Keep the White screen on the other side of the lens.
  6. Now, Light the candle and move the screen, so that you get a sharp image of the object on the screen.
  7. When you get the sharp image, stop moving the screen and note down the reading from the meter scale.
  8. Now, measure the distance of image (v) from the V- stand and note it down as image distance.
  9. And measure the distance of the object (i.e., candle ) from the V-stand and note it down as Object distance.
  10. Repeat the experiment by keeping the candle at various distances like 10cm, 20 cm etc.,
  11. Measure the object distance and image distance on each case and note it down in the table.
  12. Now, find out the focal length using the formula 1/f= 1/v-1/u.
  13. You will find that the focal length is equal in all cases.
  14. The value of ‘f’ is the focal length of the convex lens.


  1. No obstacle should be there in the path of the lens
  2. Record the image distance when you get a sharp and clear image on the White screen.
  3. Fix the lens vertically on the V-stand
  4. The base of the lens and white screen should be in line with the measuring scale.

Suggested Projects: