MCQ
The distance between object and its two times magnified real image as produced by a convex lens is $45 \mathrm{~cm}$. The focal length of the lens used is___________ $\mathrm{cm}$.
- ✓$10$
- B$4$
- C$13$
- D$85$
✓
Answer
Correct option: A.
$10$
a
$ \frac{v}{u}=-2 $
$ \frac{v}{u}=-2 $
$ v=-2 u \ldots(i) $
$ v-u=45 $ $...........(ii)$
$ \Rightarrow u=-15 \mathrm{~cm} $
$ v=30 \mathrm{~cm} $
$ \frac{1}{f}=\frac{1}{v}-\frac{1}{u} $
$ f=+10 \mathrm{~cm}$
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