(a) A plano-convex lens of refractive index $\mu_1$ is placed coaxially in contact with a biconcave lens of refractive index $\mu_2$ as shown in the figure. All curved faces are of radius of curvature $R$ each. Obtain the expression for the focal length of the combined lens. OR (b) A convex lens is kept coaxially on the left side of a concave mirror at a distance of $5f$ from it where $f$ is focal length of each of them. An object is kept at a distance of $2f$ on the left side of the convex lens. Draw the ray diagram showing the formation of the image by the combination. Find the distance of the final image from the mirror.