HC Verma Chapter 18 Problem 8 – Lens maker’s equation – biconvex lens

Given

R1=20 cm, R2=-20 cm, n=1.5

Find

Focal length f

Formula

1/f = (n-1)(1/R1 – 1/R2)

Solution

1/f = 0.5 x (1/20 + 1/20) = 0.5 x 0.1 = 0.05; f = 20 cm

Answer

f = 20 cm

Diagram / Concept

Both surfaces contribute equally for symmetric biconvex lens

Notes

R1 > 0 (convex toward incoming light), R2 < 0