Optics 3 Question 34
34. 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: 34. 6
Solution:
- $\frac{1}{v}-\frac{1}{u}=\frac{1}{f} \quad$ or $\quad \frac{u}{v}-1=\frac{u}{f} \quad$ or $\quad \frac{u}{v}=\Big(\frac{u+f}{f}\Big)$
$$ \therefore m=\frac{v}{u}=\Big(\frac{f}{u+f}\Big) \quad \Rightarrow \quad \frac{m _{25}}{m _{50}}=\frac{\frac{20}{-25+20}}{\frac{20}{-50+20}}=6 $$
$\therefore$ Answer is 6 .
