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

Answer

6

Explanation

When object distance is `25` .
`(1)/(v)-(1)/(u)=(1)/(f)`
`(1)/(v)-(1)/((-25))=(1)/(20) rArr v=100 cm` .
`m_(25)=(v)/(u)=(100)/((-25)) =-4 `.
When object distance is `50` .
`(1)/(v)-(1)/((-50))=(1)/(20) rArr u=(100)/(3) cm`
`m_(50)=(3)/(-50)=-(2)/(3)`
Alternate:
`(m_(25))/(m_(50))=((f)/(20-25))/((f)/(20-50))=(-30)/(-5)=6`