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A thin converging lens has a focal length 'f' in air. If it is completely immersed in a liquid, briefly explain, how the focal length of the lens will vary ? |
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Answer» Solution :The focal length of a CONVERGING lens is given by: `1/f = (n/n_(m)-1) (1/R_(1)+1/R_(2))` where n = refractive index of lens and `n_(m)` =refracting index of the surrounding medium. Hence in AIR `1/f = (n-1) (1/R_(1) + 1/R_(2))` and in a given liquid medium `=1/f. = (n/n_(m)-1) (1/R_(1)+1/R_(2))` `f. = ((n-1)n_(m)f)/(n-n_(m))` Following two cases are important here : (i) If `n_(m) lt n`, then `f.`is +ve and has a value greater than f i.e., focal length of the lens increases when immersed in given liquid medium but even now the lens behaves as a converging lens. (ii) If `n_m GT n`, then f. is -ve i.e., the lens will BEGIN to behave as a diverging lens. |
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