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Calculate the focal length of a biconvex...

Calculate the focal length of a biconvex lens in air if the radii of its surfaces are 60 cm and 15 cm. Refractive index of glass = 1.5

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Consider a light ray going through the lens as shown. It strikes the convex side of 60 cm radius and concave side of 15 cm radius while coming out.
`R_(1)=+60cm impliesR_(2)=-15cm`
`therefore 1/f[mu/mu_(0) -1][1/R_(1)-1/R_(2)]` or, `1/f=[1.5/1 -1][1/60 + 1/15] implies f=+24cm`
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