Doppler Redshift

Similar to sound waves, when a light source is moving away from an observer, the received frequencies of the light waves are decreased. Since the frequencies are represented by the colors of light, with red being the lowest and violet the highest, the observed red-shifted light from a remote celestial body convinces us that our universe is expanding. Based on the Doppler effect, radar that transmits and receives a beam of electromagnetic wave reflected by a moving body, is used to measure the velocity of airplanes and automobiles.