Doppler Effect

The change in the observed frequency of an acoustic or electromagnetic wave due to relative motion of source,observer or both is called Doppler effect. As the source moves towards the observer the frequency increases or wavelength decrease and if the source is moving away then the frequency decrease or wavelength increase.

The movement of the source alters the wavelength and the received frequency of the waves ,even though the source frequency and wave velocity are unchanged. The Doppler effect can be observed for any type of waves like water waves, sound waves, light waves, etc.

There are many practical applications of Doppler effect ,let us look at few specific examples.

Astronomy

Astronomers have found that Universe is expanding by Doppler effect . When a star or a galaxy moves away from us the frequency decreases or wavelength increases.This change to longer wavelengths towards the red end of the visible spectrum for visible wavelengths is called a redshift. Conversely, if the source is moving towards the observer, there is a change to shorter wavelengths, i.e. a blueshift. Most of the galaxies appeared red shifted and hence moving away from us .This is the basis for the conclusion that the Universe is expanding and was one of the greatest intellectual revolutions of the twentieth century.

Radar Gun

Police use Radar gun to measure the speed of moving vehicles ,the radar gun function on the principle of Doppler effect .The radar gun includes a radio wave transmitter integrated with a radio wave receiver. The gun emits radio waves at a particular frequency so that the radio waves travel via the air and bounce off the moving vehicles. Some of these reflected waves will return back to the radar gun, and the waves that are reflected from the vehicles undergo a Doppler Shift.This Doppler shift in turn causes a change in the frequency of the radio waves. The radar gun makes use of this frequency change to measure the speed of the vehicle.

Ocean Depth 

The depth of a ocean can be determined by using Doppler effect ,the sound waves are transmitted from the ship towards the ocean floor using a sonar(sound navigation ranging) and they are reflected from the ocean floor back to the ship.The depth can be 

calulated using the formula

        v X t

   d= --------

            2

where v is the velocity of sound and t is time taken from the ship towards the ocean floor and back to the ship.