MAGNIFICATION

(i) Magnification in Mirror

In new Cartesian sign convention, we define magnification is such a way that a negative sign (of m) implies inverted image and vice-versa. A real image is always inverted one and a virtual one is always erect. Keeping these points in mind and that the real object and its real image would lie on the same sides in case of mirror and on opposite sides in case of lenses, we define m as in case of reflection by spherical mirror as:

POWER OF A LENS

If f is in metres then the power P of the lens in dioptres is given by,  P = F^-1
If two lens in are separated by a distance d then power of combination of lens is 
 P = P₁ + P₂ - (dP₁P₂ / m)
Where P1 and P2 are optical powers of the two lenses, and μ is the refractive index of the medium in between them.