HC Verma Chapter 18 Problem 59 – Thick lens – focal length approximation

Given

Biconvex lens R1=20, R2=-30, thickness 5 cm, n=1.5

Find

Approximate focal length

Formula

1/f = (n-1)(1/R1 – 1/R2) (thin lens approximation)

Solution

1/f = 0.5(1/20 + 1/30) = 0.5 x 5/60 = 1/24; f = 24 cm

Answer

f approx 24 cm (thin lens estimate)

Diagram / Concept

Thin lens formula gives starting estimate; exact requires matrix optics

Notes

For t much less than f, thin lens approximation is adequate