Question

When is the power of the lens (a) positive (b) negative?

Answer 1

object is placed in the 1st aperture

Answer 2

Focal length of a converging i.e. convex lens is considered positive and focal length of a diverging lens i.e. concave lens is negative. Therefore power of a convex lens e is positive and that of a concave lens e is negative. Hence, if power is negative this means focal length is negative and we have a diverging lens.