Question

In mirror, magnification m = h'/h = −v/u. In lens, magnification m = h'/h = v/u. Why there is a difference in both the cases (in mirror v has negative sign while in lens v has positive sign)?

Answer 1

In Negative concave and convex positive

Answer 2

Magnification may be negative or positive in convex lens.

Magnification is always positive in concave lens .

                         Note :

• f = focal length

• v = distance of image .

• u = distance of object .

Magnification formula :

M = v / u

Concave lens :

f = negative

u = negative

v =  negative

M = - v / - u = v / u

M is positive

Convex lens

f = positive

u = negative

v = positive for real image ,

v = negative for virtual image .

M may be negative or positive

The optical centre is chosen as the origin of the coordinate system.

With respect to optical centre :

The part which are above are positive, below negative.

The part which are right are positive while left is negative.

This is just like the normal x-axis and y-axis in the co-ordinate system .