Focal length of convex lens having refractive index 1.5 is 2 cm. If this lens is dipped in a liquid having refractive index 1.25 its focal length will be ...... cm.

Options

A. 10
B. 2.5
C. 5
D. 7.5

Answer

C

Explanation

`(f_a)/(f_l)=((n_g)/(n_l)-1)/((n_g)/(n_a)-1)=(n_g-n_l)/(n_g-n_a)xx(n_a)/(n_l)`
`(2)/(f_1)=(1.5-1.25)/(1.5-1.00)xx(1.00)/(1.25)`
`thereforef_l=(2xx1.25xx0.5)/(1.0xx0.25)`
`thereforef_l=(1.25)/(0.25)thereforef_l=5` cm