The image of a candle flame formed by a lens is obtained on a screen placed on the other side of the lens. If the image is three times the size of the flame and the distance between lens and image is `80 cm,` at what distance should the candle be placed from the lens ? What is the nature of the image at a distance of `80 cm` from the lens ?

Explanation

The image is real as only the real image can be taken on the screen.
Here, image distance v = +80 cm
Magnification, m = -3
Object distance , u = ?
Since, magnification,
`m=(v)/(u)implies-3=(80)/(u)`
`u=(-80)/(3)cm`
Nature of image
`to` Real, inverted
`to` magnified
`to` formed beyond 2F
Using lens formula. we have
`(1)/(v)-(1)/(u)=(1)/(f)`
`(1)/(f)=(1)/(80)-(3)/(-80)=(4)/(8)=(1)/(20)`
`impliesf=20cm`
Positive focal length denotes that lens is convex.