NEET Physics Wave Mechanics NEET PYQ-Wave Mechanics

The fundamental frequency in an open organ pipe is equal to the third harmonic of a closed organ pipe. If the length of the closed organ pipe is 20 cm, the length of the open organ pipe is                                          [NEET - 2018]

A) 12.5 cm

B) 8 cm

C) 13.2 cm

D) 16 cm

Correct Answer: C

Solution :

[c] For closed organ pipe, third harmonic

\[\text{=}\frac{3v}{4l}\]

For open organ pipe, fundamental frequency

\[=\frac{v}{2l'}\]

Given,

\[\frac{3v}{4l}=\frac{v}{2l'}\]

\[\Rightarrow l'=\frac{4l}{3\times 2}=\frac{2l}{3}\]

\[=\frac{2\times 20}{3}=13.33cm\]