Q8 · Optics

Problem

Bi-convex lens from two plano-convex lenses (n₁=1.5, n₂=1.2, same R=14 cm). Object at 40 cm. Image distance?

Options

Option	Value
(A)	−280 cm
(B)	40 cm
(C)	21.5 cm
(D)	13.3 cm

1. Individual focal lengths

Compute 1/f for each lens, then 1/f_eq = 1/f₁ + 1/f₂.

$$\frac{1}{f_{eq}} = \frac{1}{f_1} + \frac{1}{f_2}$$

2. Image

1/v − 1/u = 1/f_eq gives v = +40 cm.

$$v = 40\,\text{cm}$$

Hints

• Find equivalent focal length of combination.

Answer

(B)