Linear magnification of a convex lens, `m=("size of image" (h_(2)))/("size of object" (h_(1)))=(v)/(u)`
Talking `h_(1)` as positive, magnification (m) will be positive, when `h_(2)` is positive, i.e., image is erect and virtual. This happens when object distance (u) from the lens is less than focal length `(f =50 cm)` of the convex lens.
Again magnification (m) will be negative, when `h_(2)` is negative, i.e., image is inverted and real. This happens when object distance (u) from the lens is more than the focal length `(f =50 cm)` of the lens.