(a) The linear magnification of a concave lens is always positive. Why?
(b) The linear magnification of a convex lens may be positive or negative. Why ?

Explanation

(a) This is because a concave lens forms a virtual and erect image for any position of the object.
(b) This is because image formed by a convex lens may be real and inverted for some positions of the object and image formed may also be virtual and erect for some other positions to the object.