Question

why is magnification in terms of mirrors -v/u whereas in lenses it is v/u ?

Answer 1

if we derive the expressions for magnification, you will get a nagative sign in the case of mirrors and a positive sign in the case of lenses. This is in agreement with the fact that all real images are inverted and all virtual images are erect.

For example, in the case of convex mirrors, always virtual images are formed behind the mirror. When the object is on the left side of the mirror, image is on the right side of the mirror. Hence u is negative and v is positive. Magnification m = −v/u is positive, and the image is erect and virtual.

Similarly you can think of other examples of concave and convex lenses and concave mirrors.