Question

A thin convex lens of power 5 D is kept in contact with a thin concave lens of power 8 D. Calculate the focal length of the combination.

Answer 1

Given: Power of convex lens, P₁ = +5 D

Power of concave lens, P₂ = −8 D

Formula: Power of the combination (P)  = P₁ + P₂

= (+5 D) + (−8 D)

= −3 D

The focal length (f)  is the reciprocal of the power (P):

`f = 1/P`

= `1/(-3)`

= −0.333 m

= −0.333 × 100

= −33.33 cm