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 a refractive index 1.50. If
the liquid has refractive index 1.60, the
focal length of the system is

Answer 1

Explanation:

f = R / 2(n -1)

for convex lens

f = R / 2(1.5 -1)

f=R

for plano concave

f' = R / (n -1)

f' = R / (1.6 -1)

f' = f/ 0.6

Net f

1/f^ = 1/f - 2/f'

1/f^ = - 0.2/f = -0.2/ 20

f^ =  -100cm