Doppler’s Effect

Doppler effect also known as doppler shift is a phenomenon which is observed whenever the source of waves is moving with respect to an observer. When an ambulance crosses you with its siren blaring is a common example of the doppler effect.

Doppler Effect Explanation

The Doppler effect is a very important phenomenon in different scientific disciplines which includes planetary science. The doppler effect or the doppler shift explains the frequency changes of any sound or light wave which is generated by a moving source with respect to an observer.

Doppler effect in physics is the increase or decrease in the frequency of sound, light, or other waves when the source and observer move towards or away from each other.

Waves which are emitted by a source traveling towards the observer gets compressed. Whereas the waves which are emitted by a source travel away from the observer get stretched. The Doppler Effect (Doppler Shift) was first proposed by Christian Johann Doppler in 1842.

Examples of Doppler Effect 

When two people P and Q are standing on the road and a car is moving towards P. Then the person P hears more sound or the magnitude of the sound is more as compared to person B. It is because the car is moving away from the person B. Hence, the frequency waves is high for person P.

Doppler Effect Formula

The Doppler effect is the apparent change in frequency of sound waves due to relative motion between the sound source and the observer. We can derive the apparent frequency in the Doppler effect using the following equation:

f’=(VV0)/(VVs)f

Here, 

f’= Observed frequency

V = Velocity of sound

V0= object velocity

Vs= source velocity

f = actual frequency

While there is only one Doppler effect equation, the previous equation changes in various situations depending on the speeds of the viewer or sound source.

Equation of Doppler Effect in various situations

When source is moving towards observer which is at rest

In this situation the velocity of observer is zero hence V0 is zero. Putting the value of V0in the Doppler effect equation then we get

f’=(V/(V-Vs))f

Here, 

f’= Observed frequency

V= Velocity of sound

 V0= object velocity

Vs= source velocity

f=actual frequency

When source is moving away from the observer which is at rest

In this situation the velocity of observer is zero hence V0 is zero. And the source is moving away from the observer hence the velocity of source becomes negative. Now, put these values in the Doppler effect equation then we get

f’=V/(V-(-Vs))f

Here, 

f’= Observed frequency

V= Velocity of sound

V0= object velocity

Vs= source velocity

f=actual frequency

When observer is moving towards the source which is at rest (that is stationary source)

In this situation the velocity of source is zero hence Vs is zero. Putting the value of Vs in the Doppler effect equation then we get f’=((V+V0)/V)f

When observer is moving away from the source which is at rest (that is stationary source)

In this situation the velocity of source is zero hence Vs is zero. And the observer is moving away from the source hence the velocity of observer becomes negative. Now, put these values in the Doppler effect equation then we get

f’=((V-V0)/V)f

Example: Two cars P and Q are moving towards each other with a velocity of 54 km/h. When frequency of the whistle emitted by P is 130 Hz, determine the apparent frequency of the whistle heard by the person sitting in car P. (The velocity of sound is 360 m/s).

Sol. According to question, The observer and the source are moving towards each other. Therefore,

f’=(V+V0)/(V-Vs)f

Velocity = 54 km/s = 545/18 m/s = 15 m/s

f’=(360+15)/(360-15)130

f’=375/345130

f’=141.304 Hz

Doppler effect applications

There are many applications of the doppler effect some of which we will discuss.

1. Doppler effect is used in measurement of Blood Flow.

2. The Doppler effect is also applicable in audio.

3. The Doppler effect is very useful in Satellite Communication.

4. The Doppler effect is also used in astronomy.

Limitations of Doppler effect

1. The Doppler effect is only applicable when the velocities of the sound source and the observer are much smaller than the speed of sound. 

2. The movement of both the source and the observer must be along the same straight line.

Doppler effect in light

The Doppler effect of light can be described as the obvious change of the light frequency observed by the observer due to a relative movement between the light source and the observer.

For sound waves the equations for the Doppler effect differ significantly, depending on whether it is the source, observer or air that moves. Light does not need any medium, and the Doppler effect for light travels in a vacuum is depends on the relative velocity of the observer and source.

Conclusion

Doppler effect in physics is the increase or decrease in the frequency of sound, light, or other waves when the source and observer move towards or away from each other.

Doppler Effect Formula

f’=(V±V0)/(V±Vs)f

Doppler effect is used in measurement of Blood Flow.

Doppler effect is also applicable in audio.

Doppler effect is very useful in Satellite Communication.

Doppler effect is also used in astronomy.

The Doppler effect is only applicable when the velocities of the sound source and the observer are much smaller than the speed of sound. 

The movement of both the source and the observer must be along the same straight line.