What is Doppler Effect

Doppler Effect is the apparent change in frequency of a wave caused by the relative motion of the source relative to the observer. The Doppler Effect is a physical phenomenon where an apparent wavelength shift is presented by a sound source with respect to the observer when the same source is moving. This phenomenon is named after its discoverer, Christian Andreas Doppler, Austrian physicist and mathematician who presented his first theories on the matter in 1842.

How the Doppler effect happens?

Doppler Effect 1Doppler Effect 3


Any signal which reaches the receiver, starting from a source is composed of waves. It does not matter whether it is to sound waves or light waves .The distance between the apexes of the waves is wavelength or frequencyIf transmitter and receiver move towards each other, the wavelength is apparently shortened .This is called the Doppler effect .

However, the Doppler effect that is caused by the movement of the signal source or the receiver only observed by the source and the receiver. For outside observers, the wavelength remains unchanged. An example of the Doppler effect in everyday life can be  watching  the moving objects with sources that emit sound waves.  As long as the car is traveling towards the observer, the pitch of the signal increases. Once the car has passed by the receiver  the pitch rate drops.


Where is the Doppler effect used?

In practice, there are various  applications . It is used for example to measure the distance and speed of moving objects. The notorious speed trap , for example, is based on the Doppler effect mentioned above. Also the various radar systems, the weather radar, radar detection of flying objects in the civil and military aviation, utilize  the Doppler effect. It is also used in the medical applications.

Doppler Effect

Big bang theory Doppler Effect

The idea that the universe is expanding since the Big Bang that gave birth to the universe is based on a natural phenomenon called the Doppler Effect.

 The Doppler effect and the origin of the universe

How a  simple theory like Doppler effect can be related with  the origin of the universe? Well, oddly enough, this effect was determined to formulate the theory of the Big Bang . The Doppler effect was something of vital importance in this type of discoveries related to the expansion of the universe and its constant motion. The most important scientists in the field are Edwin Hubble and his colleague Milton Humason ,


Hubble’s Law related with the Big Bang

When these two astronomers began their arduous task, they found impressive data: redshift. Redshift means: shift toward the longer wavelengths in the spectral lines emitted by a heavenly body (caused when an object is moving away from an observation point).  They found almost all galaxies experiencing a redshift process which was considered as a  proof that the universe must be expanding. And even more importantly, the further away the galaxies were, the higher redshift they had. While neighboring galaxies show a very small  amount of red shift , in the further ones it was huge. They also discovered that there was a strong correlation between the distances and redshift intensity.

This theory gave validity to the Big Bang and helped laying the groundwork for the expansion of the universe. Studying the different intensity of redshift galaxies, Hubble measured the distance and speed of  each, and concluded that the more distant a galaxy was, the the further it is.

Such theories are known as “Hubble’s Law” , and from them we can deduce that the galaxies are receding from each other at a speed proportional to their distance. As already stated,it is fundamental to understanding the universe’s expansion. It apparently demonstrates that these expansions which we can still see are the “remains” of the big bang. However, Hubble’s law was used for various purposes, such as to make the first scientific approaches on the age of the universe, ie the period of time that has elapsed since the Big Bang to the present day. Currently, scientists think that this figure is around 13,700,000,000 years.


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