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=?
Now, magnification,
`m=v/u implies -3=(80)/(u)` or `u=(-80)/(3) cm`
Nature of image will be real, magnified, and inverted. The image will be formed beyond 2f.
By using lens formula,
`1/v - 1/u = 1/f`
`implies 1/f=(1)/(80)- (3)/(-80)=(1)/(20)` or f = 20 cm
Focal length is positive so, the lens is convex.