Question no.18. Ans(2)
$$\begin{gathered} Plane surface of plano convex lens of refractive index \text{'}\mu \text{'} and radius of curvature R is silvered and object is placed in front of curved face then focal length of lens is \\ \left(1\right) \frac{R}{2(\mu -1)} \\ \left(2\right) \frac{-R}{2(\mu -1)} \\ \left(3\right) \frac{2(\mu -1)}{R} \\ \left(4\right) \frac{-2(\mu -1)}{R} \end{gathered}$$