A) 9 : 8
B) 8 : 9
C) 1 : 1
D) 9 : 10 (Speed of sound v = 340 m/s)
Correct Answer: A
Solution :
When source is approaching the observer, the frequency heard \[{{n}_{a}}=\left( \frac{v}{v-{{v}_{S}}} \right)\times n=\left( \frac{340}{340-20} \right)\times 1000=1063Hz\] When source is receding, the frequency heard \[{{n}_{r}}=\left( \frac{v}{v+{{v}_{S}}} \right)\times n\]=\[\frac{340}{340+20}\times 1000=944\] \[\Rightarrow {{n}_{a}}:{{n}_{r}}=9:8\] Short tricks : \[\frac{{{n}_{a}}}{{{n}_{r}}}=\frac{v+{{v}_{S}}}{v-{{v}_{S}}}=\frac{340+20}{340-20}=\frac{9}{8}.\]
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