Obtain the magnification by lens after giving its definition.

Explanation

The ratio of the size of the image obtained by lens to that of the object is called magnification.


In figures (a) and (b), convex and concave lenses are shown respectively.
Object height AB = h, Image height A.B. = h. Object distance BP = u, Image distance B.P = v
Right angle triangles ABP and A.B.P are similar triangles.
`therefore(AB)/(A.B.)=(BP)/(B.P.)`
`therefore(h)/(-h.)=(v)/(u)` [`because` According to sign convention]
`therefore(h.)/(h)=v/u`
`therefore` Magnification m = `v/u`
Magnification is negative for real image and positive for virtual image.