Q. The focal length of a concave mirror is 20 cm. Where an object must be placed to form an image magnified two times when the image is real?
A) 30 cm from the mirror
B) 10 cm from the mirror
C) 20 cm from the mirror
D) 15 cm from the mirror
Explanation
(a) $m = \frac{f}{f - u}$
$- 2 = \frac{- 20}{- 20 - u}$
$- 2 = \frac{20}{20 + u}$
Or $\ 20 + u = - 10$
Or $\ u = - 30\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.