A sound wave of wavelength 32 cm enters the tube as shown in the figure. Then the smallest radius r so that a maximum of sound is heard at detector is :
A
7cm
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B
14cm
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C
21cm
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D
28cm
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Solution
The correct option is D28cm For max sound two waves need to interfere constructively. ∴ Path difference for the waves has to be integer multiple of wavelength. Path length =S1=πr Path length =S2=2r Path difference =Δx=S1−S2=πr−2r=nλ For min radius,n=1 ⇒πr−2r=1×λ ⇒r(π−2)=λ ⇒r×1.14=32 ⇒r=321.14=28cm