Dopplers Principle

Doppler's Principle. If waves on the sea succeed each other at equal intervals of one second we know that a stationary boat floating on the water is passed by sixty waves in a minute. If the boat advances towards the direction from which the waves are coming, more than sixty waves will pass it per minute. Thus, if it travels at the rate of the waves themselves, 120 waves will be received in that interval. So also if the boat retreats in the reverse direction, i.e. in the same direction as the waves are travelling, we know that fewer waves will manage to pass it per minute. In fact, not one will pass the boat if it travels at the same rate as the wave-motion itself. This principle, applicable to all cases of wave-motion, is known as Doppler's principle, and is of considerable importance. A steam-whistle on a railway-engine temits a certain note when the train is motionless. If the train be approaching the observer, more waves of sound reach his ear per second than in the first case, and the note seems of higher pitch. If the train is receding, the note is of lower pitch. Again, the light from a distant star is composed of various colours. Each of these coloms corresponds to a definite number of waves striking the retina of the observer's eye per second. If the star approach the earth, each colour is slightly changed; a slight change of a different, nature takes place when the star recedes from the earth; and by noting the discrepancy that occurs in the light from flaming sodium that reaches us from a distant star, we are able to estimate the speed at which the star is changing its distance from our solar system.