Optics Ques 83
- The focal length of a thin biconvex lens is $20 $ $cm$. When an object is moved from a distance of $25 $ $cm$ in front of it to $50 $ $cm$, the magnification of its image changes from $m _{25}$ to $m _{50}$. The ratio $\frac{m _{25}}{m _{50}}$ is
(2010)
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Answer:
Correct Answer: 83.$(6)$
Solution:
- $\frac{1}{v}-\frac{1}{u}=\frac{1}{f} \quad$ or $\quad \frac{u}{v}-1=\frac{u}{f} \quad$ or $\quad \frac{u}{v}=(\frac{u+f}{f})$
$ \therefore \quad m=\frac{v}{u}=(\frac{f}{u+f}) \quad \Rightarrow \quad \frac{m _{25}}{m _{50}}=\frac{(\frac{20}{-25+20})}{(\frac{20}{-50+20})}=6 $
$\therefore \quad $ Answer is $6$ .
BETA
