An organ pipe is in resonance with a tuning fork. What change will have to be made in the length l to maintain resonance if
(i) temperature increases
(ii) air is replaced by hydrogen
(iii) pressure is made higher ?

Explanation

Fundamental frequency of an open organ pipe is given by
`v = (upsilon)/(2L) = (1)/(2L) sqrt((gamma P)/(rho))`
(i) With the decrease in temperature, velocity of the sound decreases, so in order for v to remain constant, L must be decreased.
(ii) Pressure has no effect on the velocity of sound and hence, it does not affect resonance.
(iii) If air inside the pipe is replaced by hydrogen, then density of gas `(rho)` decreases. So, in order to keep v constant, L must be increased.