If in a planoconvex lens, the radius of curvature of the convex surface is `10cm` and the focal length is `30 cm`, the refractive index of the material of the lens will be

Explanation

`R_1=infty,R_2=-10cm,f=-30cm,mu=?`
`(1)/(f)=(mu-1)((1)/(R_1)-(1)/(R_2))`
`(1)/(30)=(mu-1)((1)/(infty)-(1)/(-10))=(mu-1)/(10)`
`mu=1+(1)/(3)=(4)/(3)`