What is Doppler Effect?

What is Doppler Effect?

Everyone has observed the phenomenon of Doppler Effect at some point. An ambulance coming towards you sounds a bit high pitched and then when it goes away from you it sounds a bit low pitched. So basically Doppler Effect is the change in frequency or wavelength due to the relative motion between source and observer. This phenomenon was named after the Austrian physicist, Christian Doppler, who proposed it in 1842 during his time at Prague Polytechnic University.

Doppler effect can be observed in sound waves as well as electromagnetic waves i.e light. The apparent change in wavelength/frequency due to the motion of source object is called as Doppler Effect. Consider a scenario where an observer is observing a moving object. If the object is moving towards the observer the wavelength is shorter due to the motion of source, and hence the frequency increases (higher pitched sound). Whereas if the object is moving away from observer the wavelength is longer as source is moving away from the observer and hence frequency decreases (lower pitched sound).

In case of light, if the object is moving towards you its called as blue shift because the wavelength reduces i.e it shifts towards blue side of the Electromagnetic Spectrum and if the object is moving away its called as red shift because the wavelength increases i.e it shifts towards red side of the Electromagnetic spectrum.

Note that blue shift and red shift doesn’t actually mean the object appears blue or red, it just means that frequency increases or decreases. A stellar object’s spectrum may be in ultraviolet region which is already beyond blue, in that case blue shift means the spectrum shifts towards the higher frequency range.

Red Shift and Blue Shift of Electromagnetic Spectrum of a Star.

 


Some applications of the Doppler Effect


  • Police radars make use of Doppler effect. The device is pointed at the target (vehicle), radio waves are emitted which hit the target and are reflected back. Depending on whether the vehicle is moving towards or away the change in wavelength is measured and instantly speed of the vehicle is calculated by the electronic circuits in the device. Such device is a good for non-intrusive way of traffic rule enforcement.
    Handheld Police Radar.
    Image Credits : stealthveil.com

  • Doppler Radars are used by Meteorologists to study the weather. Similar to Police radar it uses radio waves, they have large enough wavelength to interact with clouds and precipitation. This can be used to determine the speed of cloud and using other parameters like wind speed, temperature, air currents,etc the prediction of weather becomes more accurate.
    Doppler Radar at the National Weather Service in Dodge City, Kansas.

  • Doppler Echo-cardiogram is a device used to take images of heart. It uses sound waves which makes it relatively safe medical imaging technique. The sound waves bounce off the walls of heart and the red blood cells hence we obtain an image which helps determine the rate of blood flow and direction.
    Doppler Mitral Valve.
    Image : Wikipedia.

  • In Astronomy and Cosmology Doppler effect is used to determine if a stellar object is moving towards or away from us. It is also used to determine the distances of stellar objects. Click here to read more about determining stellar distances.
  • When a planet orbits a star, the star wobbles around the center of mass of the star planet system also called as barycenter (common center of mass for star and their planets). So the wobble means that the star moves away from us and towards us. That’s it! We can use Doppler Shift to detect exoplanets!!

Infact our sun also wobble mostly due to Jupiter.

 

Star Wobbles due to exoplanet.

To read more in detail about Doppler effect and also it’s mathematical formulation refer to this pdf.

 

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