When we hear the sound of a police car speeding past us with its siren blaring, or a train hooting past another train, we find that frequency of the sound gets higher as the car approaches us, and lower as it goes past, in spite of the fact that the actual pitch of the sound remains the same. The frequency of the sound apparently seems to rise and fall, since the sound waves reach us faster when the car approaches us, and reverse is the case when it moves farther away. The effect is known as the Doppler Effect, which has been named after the Austrian physicist, Christian Doppler, who first studied it in 1842. The Doppler Effect is thus described as the apparent change in frequency of sound, light or radio waves caused by the motion of the source, observer or medium. The basis of the ‘Doppler Effect’ is the fact that sound or light travels in the form of waves. The pitch of a sound depends on its frequency. The frequency is the number of sound waves striking the ear every second. When the source of the sound is approaching us, each wave sent out by the source has a shorter distance to travel than the wave that was sent out earlier from a longer distance. Each wave reaches the listener a little sooner than it would have, if the source had not been moving. The waves seem to be more closely spaced. Hence they seem to increase to a higher frequency or pitch as a hooting train nears us. But as the train passes the observer, each successive wave starts a little further away, and each wave seems to be longer than it would ordinarily be. Hence the pitch is lowered. The same thing happens with any other form of wave. Light also travels in the form of waves and the Doppler Effect has also been observed to occur with a source of light moving away as in the case of a distant star. The spectra of stars which move away from Earth have a decreasing frequency which is known as the red shift, while stars which are approaching Earth with an increasing frequency, have spectra with a blue shift. A shift in the pattern of lines observed in the spectrum helps astronomers to find out the direction and speed of the star movement. The Doppler Effect is also used in radars to distinguish between stationary and moving targets. It provides information about their velocity by measuring the frequency shift between the emitted and reflected frequency, which is termed as ‘Doppler Shift’.
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