A man uses a concave mirror for shaving. He keeps his face at a distance of 25 cm from the mirror and gets an image which is 1.4 times enlarged. Find the focal lengt.1 of the mirror

Answer

C

Explanation

`u=-25cm`
`=m=(A_1B_1)/(AB)=u/v`
`rarr 1.4 =-(v)/(-25)`
`rarr 14/10=v/25`
`rarr v=(25xx14)/10=35 cm
`Now, 1/f=1/v+1/u`
`rarr 1/f=1/35-1/25
`=(5-7)/175=-2/175`
`rarr f=-87.5`
so, focal length of the concave mirror is 87.5 cm.