(a) A point object is placed in air at a distance R/3 in front of a convex surface of radius of curvature R, separating air from a medium of refractive index $n$ ($< 4$). Find the nature and position of the image formed. OR (b) In Young's double slit experimental set-up, the intensity of the central maximum is $\text{I}_0$. Calculate the intensity at a point where the path difference between two interfering waves is $\lambda/3$.