Doppler Effect in Light | Redshift and Blueshift

Doppler Effect in Light

We are already familiar with the Doppler effect in sound i.e., the apparent change in frequency of sound due to the relative motion between the source and the listener in a medium. It is named so, because of Doppler who was the first to have stated a theory in the case of sound waves in the year, 1842. He pointed out the relevance of the phenomenon in the case of light.


If the source of light (source of em waves) of frequency f is stationary and the observer moves towards the source with a velocity v (v << c, the velocity of light), the apparent frequency of light as observed by the observer,


f‘ = f(1 + v/c)


The above equation is also valid in the case when the source moves towards the stationary observer. If the source moves away from the stationary observer or the observer moves away from a stationary source,


f‘ = f(1 - v/c)


Doppler effect in sound is asymmetric whereas the Doppler effect in light is symmetric. It means that the apparent frequency of sound when the source is approaching a listener at rest and the apparent frequency when the listener is approaching the source at rest are different. But, for light, the source approaching the observer and observer approaching the source exhibit exactly the same Doppler change in the frequency.


Doppler effect is a convenient tool to estimate the speed and direction of motion of stars, planets etc. in our universe relative to us. In general, if the wavelengths of light received from these objects shift towards the red end of the spectrum, these objects are moving away from the earth. This is known as Redshift. If the wavelengths of spectral lines emitted by these objects shift towards the violet end of the spectrum, these objects are moving towards the earth. In the case of stars, the red shift is observed. This shows that stars are moving away from the earth. By finding the shift, the velocity of these heavenly bodies can be estimated. When waves are received from a source which is moving in the direction of the observer, then there would be an apparent decrease in the wavelength. This is referred to as Blueshift.


The Doppler effect finds application to estimate the speed of aeroplane and automobiles, track artificial satellites, estimate the velocity and rotation of the sun etc.  

Sreejith Hrishikesan

Sreejith Hrishikesan is a ME post graduate and has been worked as an Assistant Professor in Electronics Department in KMP College of Engineering, Ernakulam. For Assignments and Projects, Whatsapp on 8289838099.

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