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 magnification of its image changes from `m_(25)` to `m_(50)`. Find the ratio of `m_(25)/m_(50)`

Explanation

Since, magnification, `m = f/(f + u)`
`rArr m_(25) = 20/(20-25)= -4`
Similarly, `m_(50) = 20/(20-50) = -2/3`
Therefore, `m_(25)/m_(50) =(-4) (-3/2) = 6 rArr m_(25)/m_(50) =6`