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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?


Given data,
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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