HC Verma Chapter 18 Problem 11 – Two lenses in contact

Given

f1=20 cm, f2=30 cm in contact

Find

Equivalent focal length F

Formula

1/F = 1/f1 + 1/f2

Solution

1/F = 1/20 + 1/30 = 5/60; F = 12 cm

Answer

F = 12 cm

Diagram / Concept

Powers add: P = P1 + P2

Notes

Equivalent lens is converging (F > 0)