Question

A convex lens made of material of refractive index 1.5 and having a focal length of 10 cm is immersed in a liquid of refractive index 3.0. The lens will behave as ______.

Options

• converging lens of focal length 10 cm. 

• diverging lens of focal length 10 cm. 

• converging lens of focal length 10/3 cm. 

• diverging lens of focal length 30 cm.

Answer 1

A convex lens made of material of refractive index 1.5 and having a focal length of 10 cm is immersed in a liquid of refractive index 3.0. The lens will behave as diverging lens of focal length 10 cm. 

Explanation:

`1/"f"_"a"` = (1.5 - 1) `(2/"R")`

f_a = R = 10 cm 

when dipped in liquid

`1/"f"_"L" = (1.5/3-1)xx2/"R"`

`1/"f"_"L" = -1/"R"`

⇒ f_L = – R = – 10 cm

Diverging lens of focal length 10 cm.