An equiconvex lens with radii of curvature of magnitude R each,is put over a liquid layer poured on top of a plane mirror.A small needle ,with its tip on the principal axis of the lens,is moved along the axis until its inverted real image coincides with the needle itself.The distance of the needle from the lens is measured to be a as shown in Fig.2.53.

On removing the liquid layer and repeating the ezperiment,the distance is found to be b as shown in Fig.254 .Gien that tow values of distances meeawured reepresent the focal length values in the two cases, obtain a formula for the refractive index of the liquid.
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