Question

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

Answer 1

Correct Answer - B
`1/0.2=(1.5-1)(1/R_1-1/R_2)…….(i)`
`1/-0.5=(1.5/mu-1)(1/R_1-1/R_2)…..(ii)`
Here, `mu=`refractive index of medium or liquid.
Dividing Eq.(i) by Eq.(ii), we get
`-5=1/((1.5//mu)-1) or mu=15/8`