A) 2.25 mm
B) 4.5 mm
C) 2 mm
D) 4 mm
Correct Answer: A
Solution :
\[\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\cdot f=+\,12,\]\[u=-\,20\,cm.\] \[dv=dv\left( \frac{{{v}^{2}}}{{{u}^{2}}} \right)\] \[=1\times \frac{{{30}^{2}}}{{{20}^{2}}}=9\text{/}4\,mm=2.25\,mm\]
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