To determine the focal distance and the power of a thin lens.


  • To learn the difference between virtual and real images
  • To be able to determine the focal difference of a thin lens by measuring the distance from the object to the lens and the the distance from the image to the lens


  • Light source
  • Lens kits including optics bench, object, thin lens and a screen


The focal distance of a thin lens is determined by the lens equation:


Here d1 and d2 are the distance from the object to the lens and the image to the lens. The focal distance is f.

The power of a lens is given by:

P = \frac{1}{f}

In order for the power to be in Diopter, the focal distance f must be in meter.

Experimental Setup

Place the object onto the light source itself (it should snap magnetically). Place the lens and the screen on two place holders. Move the lens and the screen until you create a sharp image.

Here is how it looks. 

Note. Try to get your image as large as possible. The smaller the image and the distances, the larger the inaccuracy in your measurements. Do you know why?

Experimental Activity

Determine the focal distance of the lens and its power. Compare with the value given by the manufacturer which is written on the lens itself.

Questions and Assignments

  1. What is the percent difference between your value and the manufacturer value.
  2. Find the focal distance of a diverging lens (the one with the negative focal distance)