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The radii of curvature of the two surfaces of a lens are `20cm` and `30 cm` and the refractive index of the material of the lens is `1.5`. If the lens is concave`-` convex, then the focal length of lens isA. `24 cm`B. `10 cm`C. `15 cm`D. `120 cm` |
Answer» Correct Answer - D `(1)/(f)=(mu-1)((1)/(R_(1))-(1)/(R_(2)))` `(1)/(f)=(1.5-1)((1)/(20)-(1)/(30))` `(1)/(f)=0.5((30-20)/(600))` or `(1)/(f)=(1)/(2)xx(10)/(600)=(1)/(20)` or `f=120cm` |
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