Question

What is the magnification formula?

Answer 1

The magnification formula is:
M = H (i)/H (o) = -D (i)/D (o)
In other words, the ratio of the image distance and object distance to the ratio of the image height and the object height.

Answer 2

Answer: $m = -\dfrac{d_{i}}{d_{o}}$,$m = \dfrac{h_{i}}{h_{o}}$ and $m = -\dfrac{v}{u}$,

Explanation:

Formula of magnification for lens

Magnification is the ratio of the distance of the image from the lens and distance of the object from the lens.  

$m = -\dfrac{d_{i}}{d_{o}}$

Magnification is the ratio of the height of the object and height of the image.

$m = \dfrac{h_{i}}{h_{o}}$

Negative sign represent the inverted image and positive sign represent the virtual image.

Formula of magnification for mirror

Magnification is the ratio of the distance of the image and distance of the object.

$m = -\dfrac{v}{u}$