The focal length of a thin biconvex lens is `20 cm`. When an object is moved from a distance of `25 cm` in front of it to `50 cm`, the magni-fication of its image changes from `m_(25) to m_(50)`. The ratio `(m_(25))/(m_(50))` is.

Explanation

6 Given `f=+20cm` Also`m=(f)/(f+u)`
`:'``(m_(25))/(m_(50))=((20)/(20-25))/((20)/(20-5))=(-30)/(-5)=6`