The Doppler effect is applicable for

An observer moves towards a stationary source of sound with a speed $\frac{1}{5}^{\text{th}}$ of the speed of sound. The wavelength and frequency of the waves emitted by the source are $\lambda$ and $f$ respectively. The apparent frequency and wavelength heard by the observer are respectively,

When a train approaches a stationary observer,the apparent frequency of the whistle is $n'$ and when the same train recedes away from the observer,the apparent frequency is $n''$. Then,the apparent frequency $n$ when the observer moves with the train is

Two passenger trains moving with a speed of $108\, km/h$ cross each other. One of them blows a whistle whose frequency is $750\, Hz$. If the speed of sound is $330\, m/s$,then the passengers sitting in the other train,after the trains cross each other,will hear a sound whose frequency is .... $Hz$.

$A$ bus is moving with a velocity of $5 \,m/s$ towards a wall. The driver blows the horn of frequency $165 \,Hz$. If the speed of sound in air is $335 \,m/s$, then after reflection of sound wave, the number of beats per second heard by the passengers in the bus will be

$A$ whistle $S$ of frequency $f$ revolves in a circle of radius $R$ at a constant speed $v$. What is the ratio of maximum and minimum frequency detected by a detector $D$ at rest at a distance $2R$ from the center of the circle as shown in the figure?