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A double convex lens is a converging lens.

[link] shows a convex lens. Light rays traveling through a convex lens are bent towards the principal axis. For this reason, convex lenses are called converging lenses.

Light rays bend towards each other or converge when they travel through a convex lens. F 1 and F 2 are the foci of the lens.

When an object is placed in front of a lens, the light rays coming from the object are refracted by the lens. An image of the object is produced at the point where the light rays intersect. The type of images created by a convex lens is dependent on the position of the object. We will examine the following cases:

  1. the object is placed at a distance greater than 2 f from the lens
  2. the object is placed at a distance equal to 2 f from the lens
  3. the object is placed at a distance between 2 f and f from the lens
  4. the object is placed at a distance less than f from the lens

We examine the properties of the image in each of these cases by drawing ray diagrams. We can find the image by tracing the path of three light rays through the lens. Any two of these rays will show us the location of the image. The third ray is used to check that the location is correct.

Experiment : lenses a


To determine the focal length of a convex lens.


  1. Using a distant object from outside, adjust the position of the convex lens so that it gives the smallest possible focus on a sheet of paper that is held parallel to the lens.
  2. Measure the distance between the lens and the sheet of paper as accurately as possible.


The focal length of the lens is                       cm

Experiment : lenses b


To investigate the position, size and nature of the image formed by a convex lens.


  1. Set up a candle, and the lens from Experiment Lenses A in its holder and the screen in a straight line on the metre rule. Make sure the lens holder is on the 50 cm mark. From your knowledge of the focal length of your lens, note where f and 2 f are on both sides of the lens.
  2. Using the position indicated on the table below, start with the candle at a position that is greater than 2 f and adjust the position of the screen until a sharp focused image is obtained. Note that there are two positions for which a sharp focused image will not be obtained on the screen. When this is so, remove the screen and look at the candle through the lens.
  3. Fill in the relevant information on the table below
Experimental setup for investigation.


Relative position of object Relative position of image Image upright or inverted Relative size of image Nature of image
Beyond 2 f         cm
At 2 f
Between 2 f and f
At f
Between f and the lens


  1. When a convex lens is being used:
    1. A real inverted image is formed when an object is placed                      
    2. No image is formed when an object is placed                      
    3. An upright, enlarged, virtual image is formed when an object is placed                      
  2. Write a conclusion for this investigation.

Experiment : lenses c


To determine the mathematical relationship between d 0 , d i and f for a lens.

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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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