# A convex lens A of focal length 20 cm and a concave lens B of focal length 5 cm are kept along the same axis with the distance d between them. If a parallel beam of light falling on A leaves B as a parallel beam, then distance d in cm will be (a). 25 (b). 15 (c). 30 (d). 50

By Shivank Goel|Updated : August 25th, 2022

We know that

A convex Lens is a lens thick at the center but thinner at the edges (both the surfaces are curved outwards).

A concave lens is a lens thin in the middle but thicker at the edges (both the surfaces are curved inwards).

In the absence of a concave lens, the parallel beam will be focused at F2 of the convex lens, i.e., at a 20 cm distance.

5 cm is the focal length of a concave lens, i.e., if the lens is placed at 5 cm from F2 towards the left, the beam will become parallel.

In the figure, the separation between the lenses is clear

d = 20 - 5 = 15 cm Therefore, the distance d in cm will be 15.

Summary:

## A convex lens A of focal length 20 cm and a concave lens B of focal length 5 cm are kept along the same axis with the distance d between them. If a parallel beam of light falling on A leaves B as a parallel beam, then distance d in cm will be (a). 25 (b). 15 (c). 30 (d). 50

A convex lens A of focal length 20 cm and a concave lens B of focal length 5 cm are kept along the same axis with the distance d between them. If a parallel beam of light falling on A leaves B as a parallel beam, then distance d in cm will be 15.

Related Questions:- GradeStack Learning Pvt. Ltd.Windsor IT Park, Tower - A, 2nd Floor, Sector 125, Noida, Uttar Pradesh 201303 help@byjusexamprep.com