The frequency of a sound wave is equal to the frequency of the source which is producing the sound wave. It has been observed that the frequency of the sound waves produced by the same source can be changed by the motion of the source, or observer, or both. This point will become more clear from the following example. Suppose we are standing near a railway track and a fast, whistling train passes by. We find that as train approaches us, the frequency (or shrillness) of its whistle increases. And when train recedes (goes away) from us, then the frequency (or shrillness) of its whistle decreases. This change in the frequency of sound of a moving object which is otherwise giving regular vibrations is called Doppler effect.
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Thus, Doppler effect describes the change in the observed frequency of a wave, arising from a relative motion of the source of waves and the observer. The change in frequency can be explained as follows : When the source of waves (here whistle of train) is moving towards us, then more sound waves enter our ear per second, and hence the frequency of whistle appears to increase. And when the source of sound waves (whistle of train) travels away from us, then lesser number of sound waves fall on our ears per second, and hence the frequency of whistle appears to decrease.
Thus, just by observing the change in frequency of sound produced by a moving object, we can find out whether it is moving towards us or moving away from us. Because if the frequency of sound (or shrillness) is increasing, the object is moving towards us, and if the frequency of sound (or shrillness) is decreasing, then the object is receding (moving away) from us. We have just described the Doppler effect for sound waves. Actually, Doppler effect is applicable to all types of waves, including light waves.
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Application of Doppler Effect to Light Waves. By applying Doppler effect to the light waves emitted by a star in a galaxy, we can find out whether the star is moving towards the earth or away from the earth. This is because if a star is moving towards the earth, then the frequency of light emitted by it will increase gradually and shift towards the violet end of the visible spectrum VIBGYOR (because violet end of the visible spectrum has higher frequency). This is called violet shift.
On the other hand, if a star is receding (moving away) from us, then the frequency of light emitted by it will decrease and shift towards the red end of the visible spectrum VIBGYOR (because red end of the visible spectrum has lower frequency). It is this shift in frequency which is called red shift. From this discussion we conclude that violet shift indicates the approaching of stars towards us whereas red shift indicates the recession of stars. E. Hubble’s observations of the red-shift in the spectrum of light coming from the far-off galaxies, led him to conclude that the galaxies are moving away from one another at a great speed.
Thus, when Doppler effect is applied to light waves coming from various stars in the galaxies, it tells us that the various galaxies are receding from us (moving away from us) with greater and greater speeds. And it was the scientist E. Hubble who carefully studied the light emitted by different galaxies and by using Doppler effect, came to the conclusion that all galaxies move away from one another continuously. This conclusion is used in the theories which have been formulated to explain the beginning of the universe.