Q. Radius of curvature of concave mirror is $40cm$ and the size of image is twice as that of object, then the object distance is
A) 60 $cm$
B) 20 $cm$
C) 40 $cm$
D) 30 $cm$
Explanation
(d) $f = \frac{R}{2} = 20\ cm,\ m = 2$. For real image; $m = - 2$
By using $m = \frac{f}{f - u},\ - 2 = \frac{- 20}{- 20 - u} \Rightarrow u = - 30cm$
For virtual image; $m = + 2$
So, $+ 2 = \frac{- 20}{- 20 - u} \Rightarrow u = - 10\ cm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.