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A convex lens of radii of curvature 20 cm and 30 cm respectively. It is silvered at the surface which has smaller radius of curvature. Then it will behave as (μg=1.5)

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Solution

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Here, μg=1.5,R1=20cm,R2=−30cm

∴1fL=(1.5−1)[120−1−30]=0.5×[560]

or fL=1205=24cm

Focal length for mirror, fm=R2=−202=−10cm

∴ Equivalent focal length

feq=−2fL+1fm=−224+1−10=−1160⇒feq=−6011cm

Hence, this system behaves like a concave mirror of local length −6011 cm.

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