A tuning fork is in unison with an air column of length 20 cm in a resonance tube experiment . If another identical resonance tube, whose length is more by 1 cm than the first tube is used , then 4 beats/second are heard with the same tuning fork. What is the frequency of the tuning fork ?

Options

A. 80 Hz
B. 84 Hz
C. 90 Hz
D. 100 Hz

Answer

B

Explanation

`V = n_(1)l_(1) = n_(2)l_(2) :. n_(1) xx 20 = n_(2) xx 21 `
` :. n_(2) 20/21 n_(1) `
As l is increased, frequency is decreased ` :. N_(1) - n_(2) = 4`
`:. n_(1) - 20/21 n_(1) = 4 :. 1/21 n_(1) = 4 :. n_(1) = 84 ` Hz