Doppler Effect

Doppler effect also termed as Doppler shift is a phenomenon which is observed whenever the source of waves is moved 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 various scientific fields which contain planetary science. The Doppler effect or the Doppler shift explain the changes of frequency of any sound or light wave which is created by a moving source with respect to an observer.

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

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

Example of Doppler Effect 

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

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:

Here, 

= Observed frequency

= Velocity of sound

= object velocity

= source velocity

=actual frequency

While there is only one Doppler effect equation, the doppler effect formula changes in various situations depending on the speeds of the observer 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 the observer is zero. Therefore, is zero. Putting the value of in the Doppler effect equation then we get

Here, 

= Observed frequency

= Velocity of sound

= object velocity

= source velocity

=actual frequency

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

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

Here, 

= Observed frequency

= Velocity of sound

= object velocity

= source velocity

=actual frequency

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

In this case, the velocity of source body is zero therefore, is zero. Putting the value of in the Doppler effect formula then we get

Here, 

= Observed frequency

= Velocity of sound

= object velocity

= source velocity

=actual frequency

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

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

Here, 

= Observed frequency

= Velocity of sound

= object velocity

= source velocity

=actual frequency

Uses of Doppler effect

There are many uses of the doppler effect, some of which are given here.

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

2. Doppler effect is also used in audio.

3. Doppler effect is also applicable in Satellite Communication.

4. Doppler effect is also used in astronomy.

Limitations of Doppler effect

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

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

Doppler effect in light

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

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

Conclusion

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

Doppler Effect Formula

Doppler effect is applicable in measurement of Blood Flow.

Doppler effect is also used in audio.

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

Doppler effect is also applicable in astronomy.