A lens placed between a candle and a fixed screen forms a real triply magnified image of the candle on the screen. When the lens is moved away form the candle by `0.8m` without changing the position of the candle, a real image one-third the size of the candle is formed on the screen. Determine the focal length of the lens.

Answer

`0.3m`

Explanation

For `A`
`(1)/(3) = (1)/(V_(A)) - (1)/(-5) rArr (1)/(V_(A)) = (1)/(3) - (1)/(5) = (2)/(15) rArr v_(A) = 7.5cm`
For `B`
`(1)/(3) = (1)/(V_(B)) - (1)/(-6) rArr (1)/(V_(B)) = (1)/(3) - (1)/(6) = (1)/(6) rArr v_(B) = 6cm`
size of image `= v_(A) - v_(B) = 1.5cm`.