Calculate two possible distances of an object from a convex lens of focal length 20 cm so as to obtain an image of double the size of the object.

Explanation

`u=?,f=20cm, m= +-2`
As `m=v/u = +-2`
`:. v= +-2 u`
From `(1)/(v) - 1/u = (1)/(f)`
`(1)/(+-2u) - 1/u = 1/20`
On solving, we get `u =- 10cm` and `u = -30cm`.
These are the possible distances of the object from convex lens.