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## QuestionPhysicsClass 12

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,
$$f=+12\;\text{cm},R_1=+10\;\text{cm},R_2=-15\;\text{cm}$$
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 $$n=1.5$$