a) To measure the speed of an automobile:
(i) An electromagnetic wave is emitted by a source attached to a police car. The wave is reflected by a moving vehicle, which acts as a moving source.
(ii) There is a shift in the frequency of the reflected wave.
(iii) From the frequency shift, the speed of the car can be determined. This helps to track the over speeding vehicles.
b) Tracking a satellite:
(i) The frequency of radio waves emitted by a satellite decreases as the satellite passes away from the earth.
(ii) By measuring the change in the frequency of radio waves, the location of satellites is studied.
c) RADAR (Radio Detection And Ranging):
(i) In RADAR, radio waves are sent, and the reflected waves are detected by the receiver of the RADAR station.
(ii) From the frequency change, the speed and location of aeroplanes and aircrafts are tracked.
(d) SONAR: In SONAR, by measuring the Q change in the frequency between the sent signal and received signal, the speed of marine animals and submarines can be determined.

(i) Let S and L be the source and the listener moving with velocities v_s and v_L respectively.
(ii) Consider the case of source and listener moving towards each other (Figure).
(iii) As the distance between them decreases, the apparent frequency will be more than the actual source frequency.

(iv) Let n and n' be the frequency of the sound produced by the source and observed by the listener respectively.
(v) Then, the expression for the apparent frequency n' is
\(n'=\left( \frac { v+{ v }_{ 1 } }{ { v-v }_{ s } } \right) \)
Here, v is the velocity of sound waves in the given medium.

(i) The listener moves towards or away from a stationary source.
(ii) The source moves towards or away from a stationary listener.
(iii) Both source and listener move towards or away from each other.
(iv) The medium moves when both source and listener are at rest

(i) SONAR stands for Sound Navigation And Ranging.
(ii) SONAR is a navigation system that sends ultra sound waves to locate objects under the water.
(iii) Use light waves of high frequency to transmit information from one place to another place, because its velocity is high and it can travel large distances in a short period of time.
(iv) However, light waves cannot not travel deep into the ocean. But Sound waves can travel more distance in sea water than light waves.
(v) Hence, sound waves are used to detect the objects in the ocean.
\(=\frac { 2d }{ t } \)= velocity of sound
(vi) The average speed of a sound wave in sea water is 1,500 ms", As the value of "t" is observed, the distance of the object can be determined.
(vii) In this way, they are able to determine the direction of the objects, as well as the distance.
(viii) Some animals in the ocean have their own natural sonar system. Dolphins and whales use the principle of SONAR to identify underwater objects.

Ear trumpet:
(i) Ear trumpet is a hearing aid, which is useful for people who have difficulty in hearing.
(ii) In this device, one end is wide and the other end is narrow.
(iii) The sound from the sources fall into the wide end and are reflected by its walls into the narrow part of the device.
(iv) This helps in concentrating the sound and the sound enters the ear drum with more intensity. and to hear the sound better.
Mega phone:
(i) A megaphone is a horn-shaped device used to address a small gathering of people.
(ii) Its one end is wide and the other end is narrow.
(iii) When a person speaks at the narrow end, the sound of his speech is concentrated by the multiple reflections from the walls of the tube.
(iv) Thus, his voice can be heard loudly over a long distance.