# Doppler Effect

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#### Doppler Effect:

• The apparent change in the frequency of the sound heard by an observer, due to relative motion between the source of the sound and the observer, is called the Doppler effect.
• Example :

#### Case – I

• When a moving train blowing whistles approaches the observer standing on the railway platform, the observer hears the sound of higher frequency than the actual frequency of the whistle.

#### Case – II

• When the moving train blowing whistles recedes away from the observer standing on the railway platform, the observer hears the sound of lower frequency than the actual frequency of the whistle. Here we should note that the frequency of sound emitted by the whistle is not changing. It is the observer who is hearing different frequencies. This effect is known as Doppler effect. #### Explanation:

• When the railway engine is at rest, then the number of waves reaching the observer are constant and is equal to the actual frequency of the horn blown by the engine.

#### Case – I:

• Consider the case that the observer is stationary on a railway platform and a train blowing whistle approaches him.
• Due to this the number of waves reaching to observer per second increases. Due to which the apparent frequency heard by the observer is greater than the actual frequency of the sound.
• The same effect is observed in all the following cases. Observer Stationary and source is moving towards the observer or Source is stationary and the observer is moving towards the source or Both the source and observer are moving towards each other

#### Case – II

• Consider the case that the observer is stationary on a railway platform and a train blowing whistle recedes away from him.
• Due to this the number of waves reaching to observer per second decreases. Due to which the apparent frequency heard by the observer is less than the actual frequency of the sound.
• The same effect is observed in all the following cases. Observer Stationary and source is moving away from the observer or Source is stationary and the observer is moving away from the source or Both the source and observer are moving away from each other

#### Applications of Doppler Effect:

• In colour Doppler sonography, the ultrasonic waves refracted from body tissues can give information about the rate of flow of various fluids including blood.
• RADAR (RAdio Detection And Ranging instrument)  is used to locate moving objects like ship, aeroplanes, tank. Using the instrument the distance and speed of moving object can be determined.
• Doppler effect is used to determine the velocity and speed of rotation of astronomical objects like a star.
• It is used to determine the speed of rotation of the sun.
• A traffic police use speed detection instrument, which works on the principle of Doppler effect and is used to determine the fast-moving vehicles.

#### Limitations of Doppler Effect:

• The Doppler effect is applicable when the velocities of the source of sound and observer are much less than the velocity of sound.
• The motion of both the observer and the source is along the same straight line.
• The medium such as air, in which the observer and source are situated at rest. If the direction of motion are different or wind is blowing, modification in formulae is required.

#### Different Formulae Used in Doppler Effect:

VL = Velocity of listener

VS = Velocity of source

V = Velocity of sound

na  = Apparent frequency

n = True Frequency • (+) sign in the numerator and (-) sign in denominator indicate that the source and listener are moving towards each other.
• (-) sign in the numerator and (+) sign in denominator indicates that the source and listener are moving away from each other.

#### Condition – I: Listener is in motion and source at rest (VS = 0)

• Listener moving towards (approaching) the source: • Listener moving away from (receding) the source: #### Condition – I: Source is in motion and Listener at rest (VL = 0)

• Source moving towards (approaching) the Listener: • Source moving away from (receding) the Listener: #### Condition – II: Both source and listener Moving:

• Source and listener approaching each other: • Source and listening receding from each other: #### Compensation For Velocity of Wind:

• If the direction of the wind is the same as the direction of the sound, then quantity ‘v’ in the formula should be replaced by (v + vw) • If the direction of the wind is opposite to that of the direction of the sound, then quantity ‘v’ in the formula should be replaced by (v – vw) • Where vis the velocity of the wind.
• If the direction of the wind is making some angle with the direction of the sound then the component of the velocity of wind in the direction of sound should be taken.

 Science > Wave Motion > You are Here