A) 4 m
B) 5 m
C) 10 m
D) 20 m
Correct Answer: C
Solution :
\[dB=10{{\log }_{10}}\left( \frac{I}{{{I}_{0}}} \right)\]; where \[{{I}_{0}}={{10}^{-12}}W{{m}^{-2}}\] Since \[40=10{{\log }_{10}}\left( \frac{{{I}_{1}}}{{{I}_{0}}} \right)\]Þ \[\frac{{{I}_{1}}}{{{I}_{0}}}={{10}^{4}}\] ....(i) Also \[20=10{{\log }_{10}}\left( \frac{{{I}_{2}}}{{{I}_{0}}} \right)\] Þ\[\frac{{{I}_{2}}}{{{I}_{0}}}={{10}^{2}}\] ....(ii) Þ \[\frac{{{I}_{2}}}{{{I}_{1}}}={{10}^{-2}}=\frac{r_{1}^{2}}{r_{2}^{2}}\]Þ \[r_{2}^{2}=100r_{1}^{2}\]Þ \[{{r}_{2}}=10m\]
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