What is Doppler Effect?
Doppler Effect
When we hear the sound of a police car speeding past us with its siren blaring or a train roaring past another tain we experience something strange happening to the pitch of the sound. The sound seems to get higher as the car approches and lower as it goes past. This despite the fact that 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 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 srtiking the ear every second. When the source of the sound is approching 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. Light also travels 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 travels away from Earth have a decreasing frequency which is known as the red shift while stars which are approching 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 Dopper effect is also used in radars to distinguish between stationary and moving targets. It is termed as Doppler shift.