Ray Optics Question 7
Question 7 - 2024 (29 Jan Shift 2)
If the distance between object and its two times magnified virtual image produced by a curved mirror is $15 cm$, the focal length of the mirror must be :
(1) $15 cm$
(2) $-12 cm$
(3) $-10 cm$
(4) $10 / 3 cm$
Show Answer
Answer: (3)
Solution:
$m=2=\frac{-v}{u}$
$2=\frac{-(15-u)}{-u}$
$2 u=15-u$
$3 u=15 \Rightarrow u=5 cm$
$v=15-u=15-5=10 cm$
$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$
$=\frac{1}{10}+\frac{1}{(-5)}=\frac{1-2}{10}=\frac{-1}{10}$
$f=-10 cm$
