Question

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 ?

Answer 1

As the image is obtained on the screen, it is real.
`:.` Magnification, `m = - 3, v = 80 cm, u = ?`
As `m = v/u :. - 3 = 80/u, u = (-80)/3 cm`.
From `(1)/(f) = (1)/(v) - 1/u = 1/80 + 3/80 = 4/80 = 1/20, f = 20 cm`.
The lens is convex and image formed at 80 cm from the lens is real and inverted.