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(C) The focal length $f$ of a lens in a medium is given by the Lens Maker's Formula:$\frac{1}{f} = (\frac{\mu_l}{\mu_g} - 1) (\frac{1}{R_1} - \frac{1}

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$^a{\mu _g} < {\,^a}{\mu _l}$

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(C) The focal length $f$ of a lens in a medium is given by the Lens Maker's Formula:$\frac{1}{f} = (\frac{\mu_l}{\mu_g} - 1) (\frac{1}{R_1} - \frac{1}{R_2})$For a concave lens,the term $(\frac{1}{R_1} - \frac{1}{R_2})$ is negative.For the lens to behave as a convergent (convex) lens,its focal length $f$ must be positive.This implies that the term $(\frac{\mu_g}{\mu_l} - 1)$ must be negative,which means $\frac{\mu_g}{\mu_l} Here,$\mu_g$ is the refractive index of glass with respect to the liquid,which is $\frac{^a\mu_g}{^a\mu_l}$.Therefore,$\frac{^a\mu_g}{^a\mu_l} < 1$,which gives $^a\mu_g < ^a\mu_l$.

$A$ glass concave lens is placed in a liquid in which it behaves like a convergent lens. If the refractive indices of glass and liquid with respect to air are $^a{\mu _g}$ and $^a{\mu _l}$ respectively,then

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