Inverse Doppler Effect

The Doppler effect is a natural phenomenon we experience in our everyday life. The phenomenon is even found useful in different situations. 

We use the Doppler effect to detect an approaching bus or a train. In the hide and seek game, we perceive the sound of our opponent because of the Doppler Effect. 

But what if the effect gets reversed? Where will we find an inverse Doppler Effect? We have answered these questions below. We have discussed the inverse Doppler effect meaning and its uses in this article.

But before proceeding, let’s take a look at the basic definitions and refresh your knowledge about the Doppler Effect.

A Brief Look at Doppler Effect and its Examples

Doppler effect, also called Doppler Shift, is experienced when a change in the frequency of the wave is produced by a source object with changing positions regarding a spectator. 

You can imagine the effect with the example of the whistling sound made by a fast-moving train. As the train approaches you, the sound of the whistle increases. As it moves past and away from you, the sound diminishes. So, as the frequency of the wave increases, its pitch also intensifies, which means the sound is shriller and as the frequency decreases gradually, the pitch recedes as well, and the sound fades away.

There are three circumstances regarding the change in frequency.

When the spectator is at rest and the source of the wave is moving

In this case, the spectator will experience a high-frequency wave from the source when the source object is moving towards it. 

The source will produce a low-frequency wave when moving away from the object. 

The following equation represents the equation.

ν = νο (1+ υs/ υ)

When the source object is at rest and the spectator is moving

The frequency of the source object will appear higher when the spectator is moving against it. 

The frequency will appear lower when the spectator moves in the same direction as the source. 

You can imagine the situation with the example of running against and towards the high-speed wind. 

Swimming against and along with the flow of water is another example. 

It is given by,

ν = To[1 – υo/(υo + υ)] 

Source object and spectator are in motion

Since the source and spectator are moving at the same time, they both will experience the same frequency. 

You can consider the example of hearing a horn in a moving vehicle while sitting in the same vehicle.

It is given by,

ν = νο [(ν + νο)/(υ + υs)

ν = observed frequency

νo = frequency measure when source and spectator were at rest

υs = velocity of the source object

υ = speed of wave when the spectator was at rest

υo = velocity of spectator

To = time period measured by the spectator at rest

What are the uses of the Doppler Effect?

The Doppler Effect is used in the following ways:

• In speed detection radars operated by police.
• In leading aircraft. 
• In measuring velocities of stars.
• In sonography to study blood flow and heart rate.
• Used echocardiogram to study heart valve pulsation and heartbeat of a foetus.

We saw that the frequency increases on moving closer to the source and decreases on moving away in all these situations. 

But what if the frequency increases on moving away from the source? 

It is the Inverse Doppler Effect.

Inverse Doppler Effect Meaning 

Consider moving towards a source producing sound waves. 

You would expect a higher frequency as the distance between you and the source increases. 

But instead, the frequency decreases as you approach. The phenomenon is known as the Inverse Doppler Effect. This effect was predicted in the 20th Century, but it was observed in 2003. 

A non-linear transmission line of magnetic nature was used for the experiment to observe the phenomenon. The magnetic nature and synthetic structure of the transmission line allow electromagnetic waves to travel through them.

During the experiment, a pulse current was passed through the line. The pulse current acted as a shockwave producing a radio frequency signal. They expected a Doppler effect, meaning the waves of the frequency are compressed when the pulse advances and the space between the waves increases when the pulse recedes. But the opposite of this was observed. The intensity of the radio frequency increased with a receding pulse. 

The phenomenon of the Inverse Doppler Shift does not occur naturally. 

Light waves can also produce the Inverse Doppler Effect meaning a high frequency is achieved as the source recedes. Materials of negative refractive indices are used to produce the effect. Since all the naturally occurring materials have refractive indices equal to or more than 1 and exhibit the Doppler Shift, artificial materials are used to produce the effect. 

Inverse Doppler effect examples include future technologies like an invisibility cloak. 

Difference Between Doppler Effect and Inverse Doppler Effect

The table below summarises the difference between Doppler Effect and Inverse Doppler Effect.

We have discussed some of the frequently asked questions below. You can gain a greater understanding of the topic with the questions.

Conclusion

The Inverse Doppler Effect is proven theoretically and experimentally. But it has not found any practical applications so far. Although it has potential usage in optics, scientists have yet to progress in this field.

So far, we have discussed the Inverse Doppler Effect and its experimental proof. We began with the definitions and examples of the Doppler Effect. Then we moved on to the Inverse Doppler effect meaning and examples.