Question

Shown in the figure here is a convergent lens placed inside a cell filled with a liquid. The lens has focal length +20 cm when in air and its material has refractive index 1.50. if the liquid has refractive index 1.60, the focal length of the system is

(a)  +80 cm

(b)  -80 cm

(c)  -24 cm

(d)  -160 cm

Answer 1

Correct option   (d)  -160 cm

Explanation :

1/f (μ₂/μ₅ - 1) (1/R₁ - 1/R₂)

1/ +20 = (1 - 5 - 1)(1/R₁ - 1/R₂)  ....(1)

1/f = (1.5/1.6 -1)(1/R₁ - 1/R₂)  ....(2)

(1)  and  (2)

f/20 = (0.5/0.1)(116) /(10)

= f = -160 cm