Question

Give an example of an experiment to determine the focal length of a concave lens.

(Note that such a lens cannot project a real image onto a screen by itself, so you will need to use a two-lens system). List all materials use and describe how/why the experiment works.

Answer 1

One way to determine the focal length of a concave lens is to combine it with a convex lens so we will have a lens combination.

Note that the power of convex lens should be high.

Material needed

Light source

Lens stand

Screen

Meter scale

A convex lens of short focal length (Higher power)

Concave lens

Procedure

1) Keep the lens combination in a lens stand

2) switch on the light source and place the lens combination in front of light source.

3) Place a screen on the other side of lens and now adjust (move) the lens combination till you get a clear image on the screen.

4) After, you have got the clear image on screen, use the meter stick and measure the distance between lens combination and light source ( this will be your object distance) and disyance between lens combination and screen (this will be your image distance)

5) Now, using the lens equation, find the foal length of the lens combination.

6) Now, as we already know the focal length of convex lens, we can use the following equation to find the focal length of concave lens

1/f = 1/f1 + 1/f2

where f is the focal length of combination, f1 is the focal length of convex lens and f2 is the focal length of concave lens.

so,

1/f2 = 1/f - 1/f1