Consider a coaxial system of two thin convex lenses of focal length f each separated by a distance d. Draw ray diagrams for image formation corresponding to an object at infinity placed on the principal axis in the following cases: (a) `d lt f`,(b)`d=f`, (c ) `f lt d lt 2f`, (d) `d=2f`, and (e) `d gt 2f`. Indicate the nature of the combination (concave, convex or plane) in each case.