CBSE Class 12 Physics Doppler Effect Theory and Examples

CBSE Class 12 Physics Doppler Effect Theory and 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.

1. INTRODUCTION

The apparent change in frequency or wavelength observed by the observer when there is a relative motion between the source and the observer, is known as Doppler Effect.
Doppler's effect takes place both in sound and light. In sound it depends on whether the source or observer or both are in motion, while in light it depends only on the fact that whether the distance between source and observer is decreasing or increasing.

2. CONDITIONS WHEN DOPPLER EFFECT IS OBSERVED FOR SOUND WAVES
(a) When the source of sound is in motion and the observer at rest.
(b) When the observer is in motion and the source at rest
(c) When the source and the observer are moving with unequal velocities.
(d) When the source and the observer and intervening medium are in motion.

3. LIMITATIONS OF DOPPLER EFFECT IN SOUND
(a) The velocity of source of sound must be less than that of sound i.e. vs < v
(b) The velocity of observer must be less than the velocity of sound i.e. v0 < v
(c) If the velocity of sound source is greater than that of sound then due to shock waves the wavefront gets distorted, consequently the change in frequency will not be observed by
the observer.

4. CONDITIONS WHEN DOPPLER EFFECT IS NOT OBSERVED FOR SOUND WAVES
(a) When the source of sound and the observer both are at rest then Doppler effect in sound is not observed.
(b) When the source and the observer both are moving with same velocity in same direction.
(c) When the source and the observer are moving mutually in perpendicular directions.
(d) When the medium only is moving
(e) When the distance between the source and the observer is constant.

5. APPARENT FREQUENCY OF SOUND HEARD BY THE OBSERVER WHEN SOURCE IS IN MOTION AND OBSERVER IS AT REST.
(1) When source is in motion and observer is at rest then the cause of apparent change in frequency is that the waves either contract or expand.
(i) When source moves towards the observer: wavelength decreases. Apparent frequency is given by

Ex.1 A jet plane emits sound with a frequency 990 cycles/sec and carries on board a source of light emitting radiation of wavelength 4500Å. If the jet plane approaches the airport with a velocity 1080 km/hr, find,
(i) the frequency of sound heard by the observer on airport. (Velocity of sound
v = 330m/s, velocity of light c = 3 × 108m/s)
(A) 10890 cycles/sec
(B) 18090 cycles/sec
(C) 10980 cycles/sec
(D) 10089 cycles/sec.

Ex.2 With what velocity should an observer approach a stationary sound source so that the apparent frequency of sound should appear double the initial frequency ?

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