A concave mirror has a focal length of 20 cm. Find the position or positions of an object for which the image-size is double of the object-size.

Answer

A, C

Explanation

For the concave mirror,
`f=-20cm`
`M=v/u=2`
`rarr v=-2u`
Ist case (virtual image)
`1/v+1/u=1/f`
`rarr 1/(2u)1/u=1/f`
`rarr 1/(2u)=1/f`
`rarr u=f/2=10cm`
`:.` The positions are 10 cm or 30 cm from the concave mirror.