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

1. 13.2 cm

2. 12.5 cm

3. 8 cm

4. 16 cm

Doubtly Asked 30 Oct 2026 NEET UG 2018 Practice test neet Save WhatsApp Report

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Answer: 1. 13.2 cm

Closed pipe: odd harmonics only, \(f_n=\dfrac{nv}{4L}\). Third harmonic: \(f=\dfrac{3v}{4\times 20}\).

Open pipe fundamental: \(f_1=\dfrac{v}{2L'}\).

Equating: \(\dfrac{v}{2L'}=\dfrac{3v}{80}\Rightarrow L'=\dfrac{40}{3}\approx 13.3\ \mathrm{cm}\).

The nearest option, and the one published solutions mark, is 13.2 cm (option 1). The paper rounds 13.33 cm to 13.2 cm.

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report

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