(ii) The radii of curvature of the faces of a double convex lens are 10 cm and 15cm. Its focal length is 12 cm. What is the refractive index of glass ?
(Refractive index of air-water = 1.33. Refractive index for ar - glass = 1.5.)

Explanation

ii) Here, we have `f=+12cm`,
`R_(1)=+10 cm, R_(2)=-15cm.`
Refractive index of air is taken as unity.
We use the lens formula of Eq. `(h_(1))/(DB)+(h_(1))/(DB)=(h_(1))/(f).` The sign convention has to be applied for `f, R_(1) and R_(2)`.
Substituting the values, we have `(1)/(12)=(n-1)((1)/(10)-(1)/(-15))`
This given `n=1.5`.