A student has obtained a point image of a distant object using the given convex lens. To find the focal length of the lens he should measure the distance between the :

• lens and the object only

• lens and the screen only

• Object and the image only

• lens and the object and also between the object and the image

B.

lens and the screen only

The correct option is B.

Here, using a convex lens the student has obtained a point image of a distant object.

The lens formula is,

 ;   v s the image distance,

u is the object distance and

f is the focal length of the lens.

We have a distant object so u = .

Thus,  .

So, to find the focal length of the lens, image distance should be known, which is the distance between the lens and the screen.