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The radii of curvature of the faces of a double convex lens are \(10\text{ cm}\)  and \(15\text{ cm}\) . Its focal length is \(12\text{ cm}\) . What is the refractive index of glass?

Answer

Given data,
\(f=+12\;\text{cm},R_1=+10\;\text{cm},R_2=-15\;\text{cm}\)
Refractive index of air is taken as unity. We use the lens formula. The sign convention has to be applied for \(f,R_1\)  and \(R_2\)  Substituting the values. we have
\(\frac 1 f=\left(n-1\right)\left(\frac 1{R_1}-\frac 1{R_2}\right)\)
\(\Rightarrow \frac 1{12}=\left(n-1\right)\left(\frac 1{10}-\frac 1{-15}\right)\)
This gives \(n=1.5\)
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