Doppler Effect

It is everyday experience that the pitch of the whistle of a fast moving train is higher when the train approaches us and is lower when the train moves away from us and we study this phenomenon of motion related frequency change under doppler effect. Let us take some more physical examples to learn it better

•  Imagine a police car equipped for catching speeders is parked alongside a highway. It sends out radio waves that strike vehicles on the highway and are bounced back to a receiver in the car. The receiver compares the frequency of the outgoing and reflected waves, converts the information to kilometers per hour, and displays the speed on a dial.  Suppose the waves hit a parked vehicle. The outgoing and reflected waves have the same frequency, because there is no relative motion between the police car and the other vehicle. If a vehicle is moving at moderate speed, there is a moderate difference in the frequencies of the waves. But a fast-moving vehicle produces a big difference. The speeder is flagged down, thanks to  Doppler's effect. Similar radar devices in airport control towers use the Doppler effect to determine the speed of aircraft in the area.
• Like sound waves or radio waves, light waves can be used to measure speed. An instrument called a spectrometer spreads light into a rainbow like spectrum of thousands of parallel lines. The spectrum ranges from purple, with the greatest wave frequency, through blue, green, yellow, and orange to red, which has the lowest frequency. Recall that the increase in frequency of the sound waves was detected as a rise in pitch when the truck approached. The wave frequency of light from some luminous object (a star, for example) similarly increases if the star moves toward us. Coupled to a telescope, a spectrometer detects this movement as a shift of the spectral lines toward the blue side. A receding star would cause a shift toward the red side. The amount of the shift is a measure of the speed with which the star is approaching or receding from the earth. Much of today's understanding of the universe is base on spectrometry. The theory of the expanding universe was confirmed by the discovery of the red shift of starlight from other galaxies, an indication that the distance between them and our galaxy is increasing.