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A thin converging lens made of glass of refractive index `1.5` acts as a concave lens of focal length `50 cm`, when immersed in a liquid of refractive index `15//8`. Calculate the focal length of converging lens in air. |
Answer» Correct Answer - `20 cm` Here, `mu_(g) = 1.5, f_(l) = - 50 cm` `mu_(l) = 15//8, f_(a) = ?` `(1)/(f_(a)) = ((mu_(g))/(mu_(a)) - 1)((1)/(R_(1)) - (1)/(R_(2)))` …(i) `(1)/(f_(l)) = ((mu_(g))/(mu_(l)) -1)((1)/(R_(1)) - (1)/(R_(2)))` …(ii) `(1)/(-50) = ((1.5)/(15//8) - 1)((1)/(R_(1)) - (1)/(R_(2)))` or `(1)/(R_(1)) - (1)/(R_(2)) = +(1)/(50) xx (5)/(1) = (1)/(10)` From (i), `(1)/(f_(a)) = (1.5 - 1)((1)/(10)) = (1)/(20)` `f_(a) = 20 cm` |
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