Question

An object is placed in front of a concave lens at a distance of 45 cm from it. If its image is formed at a distance of 30 cm from the lens, calculate the focal length of the lens.

Answer 1

Given: Object distance (u) = 45 cm    ..[Negative]

Image distance (v) = 30 cm    ...[Negative as it is a concave lens]

Using Lens Maker’s formula,

`1/f = 1/v - 1/u`

= `1/-30 - 1/-45`

= `-1/30 + 1/45`

= `-1/90`

f = −90 cm

Hence, the focal length of the concave lens is 90 cm.