Doppler Effect and Light

When we listen to moving sound sources we easily realize without seeing it that the source is moving, just by observing the sound. For example, we can easily say whether a train is approaching or departing the platform just by listening the whistle or the sound of the train. Because, when the light source is moving, an observer at rest observes different frequency of the sound than the actual frequency (frequency of the sound source). If the source is moving towards the observer, he/she observes a greater frequency whereas if the source is moving away from the observer, the observer experiences shorter frequency of the same sound wave. This means that the frequency of the sound wave depends on the observer’ motion, if there is a motion then there will be a change in frequency of the sound. This frequency shift is called the Doppler effect named after the great scientist Christian Doppler. He was the first to study the frequency shift of sound waves.

Doppler was also the first to realize that frequency of light also changes for the moving observers. We all know that colour of light totally depends on its frequency or equivalently on its wavelength. So, Doppler realized that a moving observer observes colour that differs from the colour at source (as the frequency is different for the observer). Suppose you are at rest on a railway platform and your friend (say John) is on a train is moving with a velocity v with respect to you. Now a light source is at a distance d from you and John (when you and John are at the same position). You are at rest and John is moving towards the light source. You will see that the light wave emitted from the source has a speed c (). But what about John’s calculations? Will he observe the speed of light to be greater than c? Obviously no (we know that nothing can move faster than c in vacuum). John will experience the same speed of light i.e. c. Will there be any difference between their observations? Yes, they will experience different frequencies of the same light wave. John will experience greater frequency of the light wave and so he will observe blue-shifted colour of the light wave whereas you will get the same frequency of the light wave as at the light source. The relation between the frequencies measured by you and your friend is –



where c is the speed of light and v is the speed of John with respect to you.

Application of Doppler Effect of light:

Doppler Effect of light has a tremendous use in astronomy and in our daily life. Sensors use this effect to determine one’s motion. You may have encountered with doors that open automatically when you approach. This is possible for the Doppler Effect. A sensor emits radio waves which get reflected by you and then it measures the Doppler shift and determines when to open the door. Police use Doppler Effect to measure the speed of cars. But Doppler Effect is widely used in astronomy. Doppler himself predicted that one day we would be able to measure the motion of distant stars by measuring the Doppler shift of the frequency of light emitted from them. The Doppler shift is often denoted by the red-shift number z, defined by –

From this equation we can easily determine the speed of distant stars. We can also measure rotation speed of sun and other stars using Doppler Effect.  In fact, Hubble and other scientists used Doppler Effect to determine the speed of galaxies. Using Doppler Effect, Hubble found that all the stars of the galaxies are red-shifted which means that all the galaxies are moving away from each other and therefore he concluded that the universe is expanding.