Question

(i) A screen is placed at a distance of 100 cm from an object. The image of the object is formed on the screen by a convex lens for two different locations of the lens separated by 20cm. calculate the focal length of the lens used.
(ii) A converging lens is kept coaxially in contact with a diverging lens- both the lenses being of equal focal length. What is the focal length of the combination?

Answer 1

Answer:

For the first position of the lens:

For the second position of the lens:

So,

∴

From the lens formula,

(ii) The focal length of the lenses is f and –f.

So,

∴