Question

A hollow lens is made of thin glass and in the shape of a double concave lens. It can be filled with air.
Water of refractive index 1.33 or CS2, of refractive index 1.6. It will act as a diverging lens if it is
a) Filled with air and immersed in water
b) Filled with water and immersed in CS2
c) Filled with air and immersed in CS2,
d) Filled with CS2, and immersed in water​

Answer 1

=> Given :  

Refractive index of water, μ_water  = 1.33

Refractive index of CS₂, μ_CS₂ = 1.6

Refractive index of air, μ_air = 1

=> According to Lens-makers' equation for thin lenses,

1/f = (n₂/n₁ - 1) [-1/R₁ - 1/R₂]

= - (n₂/n₁ - 1) [1/R₁ + 1/R₂]

=> we know that the focal length f is positive for converging lenses, and negative for diverging lenses.

∴ n₂ / n₁ > 1

n₂ > n₁

CS₂ has more refractive index as compare to water.

Thus, Lens should be filled with CS₂, which has more refractive index as compare to water in which lens is immersed.