Question

Define magnification produced by a mirror in terms of height of an object and image. How is it related to object and image distance. Explain why magnification is positive for virtual image and negative for real image.

Answer 1

Magnification is the ratio of height of the image(h') to the height of the object(h).
It is also equal to negative of the ratio of the image distance(v) to object distance(u).

$m= \frac{h'}{h}=-\frac{v}{u}$

Real image is always formed in front of the mirror and virtual image is formed behind the mirror. 
Object distance is always negative(according to convention). When the magnification is positive, that means image distance is positive or the image is formed behind the mirror. So it is virtual. And when the magnification is negative, that means image distance is pnegative or the image is formed in front of the mirror. So it is real.

Answer 2

Magnification= Height of image/ to the height of object= -v/u=-(distance of image)/ distance of object
m=-v/u=h2/h1
image formed always in pair of either real and inverted or virtual and erect
for memorize inverted means h2=-ve
from formula we sees that magnification is +ve
Real and inverted m=+ve
virtual and erect m=-ve