When there is a movement in the source from where the waves are being produced in respect to the observer such a phenomenon is termed as Doppler Effect. Inverse Doppler Effect is just opposite of it.

In specific words it can be defined as the Superficial difference in the frequency of a wave (light or sound) from its origin and the frequency at which it reaches the observer. This happens seemingly due to the motion of the observer and source.

When a certain thing moves in air, air surrounding that thing is displaced for the movement of the object. This displacement causes disturbances and they travel at a particular speed. This speed is the speed of Sound or wave. The characteristic difference between two consecutive waves is termed as Wavelength and the time taken by the waves that pass through air is termed as frequency.

Doppler Effect is seen in inverse. This happens because of the reflection of the wave from a retreating boundary. This phenomenon can be applied in various fields such as medical, space research, traffic technology and many more.

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## History

Back in 1968, scientist named Veselago foretold that materials that have negative fractional indices also known as metamaterials can produce Inverse Doppler Effect. He foretold this theoretically. Doppler effect can also be witnessed in electromagnetic waves by the use of transmission lines and various other materials which show this abnormal phenomenon.

Though, experimentally it can only be seen in frequencies which lie within a narrow band where there is an energy gap. If someone is willing to produce this effect in a lower frequency range they can use non-resonant materials like quasi-1D.

## Mechanism

The metamaterials used to induce Doppler effect are based on the phenomenon of Resonance such as electromagnetic materials with wires made of metals which show negative permittivity or materials that have a solid core with high density and a coating that is soft and show negative density of dynamic mass.

All the experiments that have been conducted previously showed that the metamaterials present in different unit cells resonate with their own acquired frequencies and raise no coupling effect whatsoever which in turns develops an interaction system that is quite weak. This feature helps in development of various multibands and broadbands.

## Doppler Effect Formula:

### When Sources are moving:

Suppose an object is emitting sound with some frequency, say f. In such a case the amplitude that occurs of a wave occurs at a time interval which is reciprocal of frequency i.e. t=1/f. Now if the speed of sound is c then the wavelength will be w=t*c.
Now the velocity of the source is, say v and it is moving towards the observer, then the distance covered is v*t before another maxima is emitted. Thus the w(ahead)=(c-v)*t.
We can similarly have a calculation of w(behind)=(c+v)*t.
It can thus be concluded that the change in frequency depends on the motion of direction of source. If the source is coming closer to the observer, frequency increases and Vice-Versa.

### When Source is moving towards the observer but at an angle

Suppose the observer is not moving and the source makes an angle of alpha(a) while moving towards the receiver. Then wavelength (a)= (s-v(cos a))
Thus frequency with which the sound is heard is:
w=(c/(c-v(cos a))*f

### When the speed of source is same as speed of sound

When the speed of the source becomes the same as the speed of sound or more than that the waves start piling up in the forward direction. When the speed of sound is attained all the waves merge together. This phenomenon causes a great shift in pressure due to which a loud sound can be heard known as sonic boom which is quite similar to the cracking sound of a whip.

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## Examples of Doppler Effect in day to day life

• The best example that we witness of Doppler Effect in day to day life is the sound of Siren of an Ambulance and that of Fire Engine.
• Sonar-like devices that are used in order to measure the depth of the water present at depths of the Earth.
• Devices used by Police to keep a check on over speeding vehicles.
• An example of this effect in the medical field is seen in Echocardiogram. These devices are used to detect the direction of blood flow.
• The Doppler effect is used in Satellites to receive various information.

## Limitations of Doppler Effect

• The medium in which we are observing the Doppler Effect should be still, that is there should be no wind blowing.
• Practically it is applicable only when the speeds of observer and source are less in comparison to speed of sound.

## Conclusion

The Doppler effect is mainly used to determine at what speed the object is moving. The moving object can be anything- planets in the solar system, the blood in the human body or a speeding car.

Since the Doppler effect is applicable on both light and sound scientists have used this information to conclude the expansion of the universe.

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• By whom was the Doppler Effect discovered?

The Doppler effect was discovered in 1842 by an Austrian Physicist named Christian Johann.

• Is the Doppler Effect applicable in Day to Day life? If Yes, How?

Yes, the Doppler Effect is applicable in Day to Day life. Various examples that you can observe around you include: Sirens of Ambulances and Fire Engines, In hospitals, to diagnose the blood flow and heart issues, by meteorologists to foresee a storm and by traffic police to catch speeding vehicles by their radar guns.

• How can the Doppler Effect prove the Expansion of the Universe?

The Doppler effect is used to determine the shift of planets away or towards us. This phenomenon is termed as red shift and tells us about the expansion of the universe.

• Does Gravity have the Doppler Effect or not?

Gravitational Force differs if the velocity of the objects differ for the same given distance. This proves that gravity too has the Doppler effect.

• Is the Doppler Effect applicable in both Longitudinal as well as Transverse Waves?

Yes, The Doppler Effect can be witnessed in both types of Waves.

References

[1] The Doppler effect, Matthew Schwartz

[2] Explanation of the Inverse Doppler Effect Observed in Nonlinear Transmission Lines, Alexander B. Kozyrev and Daniel W. van der Weide, Phys. Rev. Lett. 94, 203902 – Published 23 May 2005.

[4] Physics Part-II Textbook for class XII  by Anup Kumar Rajput, Shweta Uppal, Arun Chikara, Vipin Dewan and R.N. Bhardwaj