Learning Aims:
  • Understanding that the lens equation describes the position of the image, given the object position and focal length
  • Understanding that the magnification of a lens is given by the ratio of image to object distance
  • Stands to hold (1) Bulb,
  • (2) Short focal-length convex lens,
  • (3) Paper screen,
  • Metre rule,
  • Pencil
Suggestions for use:

The activity can progress in one of two directions: either the students have previously studied the lens equation and the experiment will subsequently test the law, or the students have not studied the equation and will derive it on the basis of their results.

Firstly, the students should determine the focal length of their lens or verify the value given. This can be achieved by focusing light from a distant object (the ceiling lights are a good source) onto the desk and measuring the height of the lens from the table. The students should then discuss the method by which they will investigate the relationship between object and image distance using the equipment supplied, and how they will graphically show the relationship. The teacher should guide this discussion so that students will standardise all measurements of distance to be relative to the lens.

The students can then conduct an experiment by which they fix the screen in place, adjust the position of the bulb relative to the screen, and subsequently move the lens to produce an image. They should record and tabulate their data into columns corresponding to object distance, image distance, and focal lengths. To investigate the magnification, students should also measure the width of the bulb filament they observe on the screen for each combination of image/object distance and compare this to the actual width.

The students should then plot a graph of the relationship between image and object distance. For students who have not covered the lens equation, the natural tendency will be to plot object distance versus image distance (or vice versa) which will generate a curved graph. The teacher should discuss with the students what this shape might indicate in terms of the relationship, and the students can subsequently plot the reciprocal of object distance versus the reciprocal of image distance to yield a straight line.

Possible questions:

    • If you examine the x and y intercepts of the graph, what does this relate to? (1/f)
    • What is the equation of the line if M is the slope? (M=v/u)
    • How does this slope relate to the width of the filament at each object/image combination?