Question

A biconvex lens made of a transparent material of refractive index 1.25 is immersed in water of refractive index 1.33. Will the lens behave as a converging or diverging ?

Answer 1

Ans: No, it will behave as a diverging lens.

By using thin lens maker formula,

1/fw = (ng/nm – 1)(1/R1 – 1/R2)

On using sign convention R1 = +ve, R2 = -ve and ng = 1.25 and nm = 1.33

1/fw = (1.25/1.33 – 1)(1/R1+1/R2)

(1/R1+1/R2) = +ve and (1.25/1.33 – 1) = -ve value

Hence fw = -ve. So it behaves as a diverging lens.