Question

A convex lens is placed in contact with another convex lens. The focal length of the system
(a) increases
(6) decreases
(c) remains same
(d) will become negative ​

Answer 1

Answer:

option (a) increases..

Answer 2

Given: A convex lens is placed in contact with another convex lens.

To find: The focal length of the system

Explanation: A convex lens has a positive focal length. Let the focal length of the first lens be f1 and focal length of the second convex lens be f2.

In first case when there was only one convex lens , the equivalent focal length was equal to f1

In second case, when f2 was placed in contact, the equivalent focal length which is denoted by f is:

$\frac{1}{f} = \frac{1}{f1} + \frac{1}{f2}$

$f = \frac{f1f2}{f1 + f2}$

Taking examples here, let f1= 2 cm and f2= 4 cm.

In first case, f= 2 cm

In second case, f= 2*4/2+4

= 8/6 = 4/3

Clearly, 4/3 is less than 2.

Therefore, when a convex lens is placed in contact with another convex lens the equivalent focal length decreases.