
Aim: to Measure the Focal Length of a Diverging Lens 
See The Focal Length of a Lens,
The Lens Equation 

Method 
It is assumed that you can get hold of some sort of
optical bench, something like that shown below. 
It is not so easy to measure the focal length of a
diverging (concave) lens because a diverging lens
alone cannot form a real image (on a
screen). 
In this method, the diverging lens has a little help
from a converging lens. 
First obtain an image on the screen with just converging
lens in place. 



Now add the diverging lens to the situation and see
how much it changes the position of the image 



From these observations, the focal length of the
lens can be found. 

The following ray diagrams might help. 




Notice from these diagrams that the (virtual) object
distance, u is equal to XY. 
The real image distance, v can, of course, be
measured directly. 
The lens equation can then give us the focal length. 