The magnification of an object placed it front of a convex lens of focal length `20cm` is `+2`. To obtain a magnification of `-2` , the object will has to be moved a distance equal to

Options

A. `10 cm`
B. `20 cm`
C. `30 cm`
D. `40cm`

Answer

B

Explanation

When magnification is `+2`.
`u=-x`
`v=-2x`
`f=+20`
Using `(1)/(-2x)+(1)/(x)=(1)/(20)`
or `x=10cm`
To have a magnificatio of `-2`
`u=-y`
`v=+2y`
and `f=+20`
`:. (1)/(2y)+(1)/(y)=(1)/(20)`
or `y=30cm` `:. y-x=20cm`