Question

A convex lens of focal length 'f' is placed in contact with a concave lens of the same focal length. The equivalent focal length of the combination is ______.

विकल्प

• `f`

• infinity

• `f/2`

• zero

Answer 1

A convex lens of focal length 'f' is placed in contact with a concave lens of the same focal length. The equivalent focal length of the combination is infinity.

Explanation:

Given, focal length of convex lens, f₁ = f and focal length of concave lens, f₂ = - f.

The focal length of combination, when two lenses are in contact is given by

`1/f=1/f_1+1/f_2`  ...(i)

Substituting given value in Eq. (i), we get

`=> 1/f+1/f-1/f=0=>f=oo`

Hence, the equivalent focal length of the combination is infinity.