When we hear the sound of a police car speeding past us with its siren blaring or a train roaring past another train, we experience something strange happening to the pitch of the sound. The sound seems to get higher as the car approaches and lower as it goes past. This despite the fact that the actual pitch of the sound remains the same; it just seems so because sound waves reach us faster as the car gets nearer. The effect is known as the Doppler Effect 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 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, 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. They have a higher frequency or higher pitch. As the train passes away the observer, each wave starts a little further away. Each wave seems to be longer than it would ordinarily be. Hence the pitch is lowered.
The same thing happens with any other wave motion. Light also a travel in wave motion and the Doppler Effect has also been observed where a source of light, for example, a distant star, is moving. The spectra of stars which travel 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 the astronomers to find out the direction and the speed of the star.
The Doppler Effect is also used in radars to distinguish between stationary and moving targets. It provides information regarding their velocity by measuring the frequency shift between the emitted and reflected radiation. It is termed as “Doppler Shift”.
