CBSE Class 12 Physics Doppler Effect Solved Examples

CBSE Class 12 Physics Doppler Effect Solved Examples. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

 Ex.1 The frequency of whistle of an engine appears to be (4/5)th of initial frequency when it crosses a stationary observer.I the velocity of sound is 330m/s, then the speed of engine will be :

(A) 30m/s (B) 36.6m/s (C) 40m/s (D) 330m/s.

Ex.2 Two sirens, situated at a distance of 1km, are emitting sound of frequency 330 Hz. An observer is moving from one siren towards another with a velocity of 2m/s. The beat frequency heard by the observer will be (v = 330m/s)

Ex.3 The wavelength of light received from a milky way is 0.4% higher than that from the same source on earth. The velocity of milky way with respect to earth will be -
(A) 5 × 10⁶ m/s (B) 1.2 × 10⁶m/s (3) 0.2 × 10⁶m/s (4) 2 × 10⁶ m/s

Ex.4 A train approaching a hill at a speed of 40km/hr sounds a whistle of frequency 580Hz when it is at a distance of 1km from a hill. A wind with a speed of 40km/hr is blowing in the direction of motion of the train. Find the frequency of the whistle as heard by an observer on the hill. (velocity of sound in air = 1200km/hr)
(A) 580Hz (B) 620Hz (C) 600Hz (D) 720Hz
Sol. According to Doppler's effect, the apparent frequency when both source and observer move along the same direction is

Ex.5 A table is rotating about its axis with angular speed 5 rev/sec. A source of sound of frequency 1000 sec-1 is fixed on the table at a distance 0.70m from the axis. What are the maximum and minimum frequency heard by listener at rest at a certain distance from the table? The speed of sound at room temperature is 352m/s.
(A) 966Hz, 941Hz (B) 1250Hz, 1052Hz (C) 1000Hz, 960Hz (D) 1066Hz, 941Hz.

Ex.6 A source of sound of frequency 256Hz is moving rapidly towards a wall with a velocity of 5m/s. How many beats per second will be heard if sound travels at a speed of 330 m/s by an observer behind the source.
(A) 7.7 (B) 0 (C) 7.8 (D) None
Sol. When the source S is between the wall (W) and the observer (O) For direct sound the source is moving away from the observer, therefore the apparent frequency

Ex.7 A person going away from a factory on his scooter at a speed of 36km/hr listens to the siren of the factory. If the main frequency of the siren is 600Hz and a wind is blowing along the direction of the scooter at 36km/hr Find the main frequency as heard by the person.
Sol. The speed of sound in still air is 340m/s. Let us work from the frame of reference of the air. As both the observer and the wind are moving at the same speed along the same
direction with respect to the ground, the observer is at rest with respect to the medium. The source is moving with respect to the wind at a speed of 36km/hr. i.e. 10m/s. As the source is going away from the observer who is at rest with respect to the medium, the frequency heard is ν' =v/v +vs v =340/340+10× 600 = 583Hz

Ex.8 A siren is fitted on a car going towards a vertical wall at a speed of 36km/hr A person standing on the ground, behind the car, listens to the siren sound coming directly from the source as well as that coming after reflection from the wall. Calculate the apparent frequency of the wave (a) coming directly from the siren to the person and (b) coming after reflection. Take the speed of sound to be 340m/s.

(A) 515Hz, 486Hz (B) 486Hz, 515Hz (C) 510Hz, 490Hz (D) 490Hz,510Hz

Sol. Here the observer is at rest with respect to the medium and the source is going away from the observer. The apparent frequency heard by the observer is, therefore,
The wall reflects this sound without changing the frequency. Thus, the frequency of the reflected wave as heard by the ground observer is 515 Hz

Ex.9 Two trains are moving towards each other at speeds of 72km/hr and 54km/hr relative to the ground. The first train sounds a whistle of frequency 600Hz. Find the frequency of the whistle as heard by a passenger in the second train.
(a) before the trains meet and
(b) after the trains have crossed each other The speed of sound in air is 340m/s.
(A) 666Hz, 542Hz (B) 508Hz, 662Hz (C) 662Hz, 508Hz (D) None
Sol. The speed of the first train 72km/hr = 20m/s. and that of the second = 54km/hr = 15m/s
(a) Here both the source and the observer move with respect to the medium. Before the trains meet, the source is going towards the observer and the observer is also going towards the source. The apparent frequency