Convex Lens - Image Formation and Magnification

Materials Required

  • Thin convex lens
  • a lens holder (or a stand)
  • a screen fixed to a stand
  • a meter scale
  • a small scale (15 cm)
  • lighter
  • a small candle with a stand


The Procedure

Real lab Procedure

  • Place a thin convex lens vertically on a lens holder kept on a table.
  • To determine the approximate focal length of the convex lens, focus rays from a distant object (eg: tree) using a convex lens and obtain an image of the object on a screen placed on the other side of the lens.
  • Move the screen back and forth to obtain a sharp and clear image. Measure this distance between the screen and the lens as the approximate focal length of the lens, f.
  • Light a candle using a lighter. The flame in the lighted candle is the object now in the below steps.
  • Place the lighted candle on one side of the lens such that the optical centre of the lens and the centre of the candle flame lie at the same height. Make sure the flame does not flicker and maintains its height, h. Confirm that image is clearly visible by experimenting under dim lights or in a dark place. Hence, choose an appropriate place for the experiment.
  • Choose the distance between the candle flame and optical centre of the lens to be greater than twice the approximate focal length (2f) obtained for the convex lens. Record this distance as object distance, u in the observation table.
  • Place a screen fixed on a stand on the other side of the lens and obtain a sharp image of the flame on the screen. The distance between the lens and the screen is taken as image distance v. Record this in the observation table.
  • Measure the height h’ of the flame in the image obtained on the screen. The ratio of the height of the image to that of the object is noted as the magnification, M, and should be recorded in the observation table.
  • Repeat the experiment for other scenarios in the observation table such as when the object distance is at 2f and between f and 2f. You can vary the object distance either by changing the position of the convex lens or the lighted candle.
  • Now, place the lighted candle at the focus. Observe the image formed on the wall at the end of the room. Record your observation.
  • For the 5th scenario, place the object between the focus and the optical centre. Note that no real image is obtained on the screen. Look at the candle flame through the lens and observe the nature of the image.

Simulator Procedure

  • Select the focal length of convex lens(f) from the drop down list.
  • Select the distance between lens and candle from the slider(u).
  • Drag the slider to adjust the screen position to get the clear and inverted flame image on the screen (v).
  • Click on the show result button to show the height of the candle flame formed in the screen and magnification.

Observation Table

Sl. No: Position of candle from the lens Object Distance u (cm) Image Distance v (cm) Position of image Size of the image (Diminished/ Enlarged / Same size) Magnification(M = h’ / h ) Nature of Image (Real / Virtual )
1. Beyond 2f            
2. At 2f            
3. Between f and 2f            
4. At f            
5. Between f and optical centre O            


  • A convex lens can form real and virtual images.
  • Real images are inverted while virtual images are erect.
  • Based on the position of the object, the image formed can be diminished, enlarged, or of the same size.