HC Verma Chapter 18 Problem 12 – Two lenses separated

Given

f1=20, f2=30, separation d=10 cm

Find

Equivalent focal length

Formula

1/F = 1/f1 + 1/f2 – d/(f1 f2)

Solution

1/F = 0.05 + 0.0333 – 10/600 = 0.0667; F = 15 cm

Answer

F = 15 cm

Diagram / Concept

Separation reduces combined power

Notes

Position of equivalent lens also shifts