A thin equi-convex lens is made of glass of refractive index `1.5` and its length is `0.2 m.` If it acts as a concave lens of `0.5 m` focal length when dipped in a liquid, the refractive index of the liquid is

Options

A. `17//8`
B. `15//8`
C. `13//8`
D. `9//8`

Answer

B

Explanation

`(f_(1))/(f_(a)) = (n_(ga) - 1)/( n_(gl) - 1) rArr (-0.5)/(0.2) = (1.5 - 1)/(n_(gl) - 1)`
`rArr n_(gl) = 0.8 = (4)/(5)`
`rArr (n_(ga))/(n_(la)) = (4)/(5) rArr (1.5)/(n_(la)) = (4)/(5)`
`rArr n_(la) = (15)/(8)` .