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Optics
 
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 X-Y.
The real image distance, v can, of course, be measured directly.
The lens equation can then give us the focal length.
 
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