A convax lens of glass ,refractive index`1.5`,has both surfaces of same radius of curvature `R.`On immersion in a medium of refractive index`1.75`,it wuill behaves as a
(A)convergent lens of focal length `3.5R`(B)convergent lens of focal length `3.0R`(C)divergent lens of focal length `3.5R`(D)divergent lens of focal length `3.0R`

Answer

A

Explanation

`(1)/(F')=(._(l)mu(g)-1)((1)/(R_(1))-(1)/(R_(2)))`
`=((._(a)""mu_(g))/(._(a)""mu_(l))-1)((1)/(R)-(1)/(-R ))`
`((1.50)/(1.75-1)(-(2)/(R ))=((6)/(7)-1)((-2)/(R ))`
`(1)/(F')=(-(1)/(7))(-(2)/(R ))`
`(1)/(F')=(2)/(7R)`
`F'=(7)/(2)"R"`
`F'=3.5R`