Observations 4

Astronomy Cast - Ep. 165: The Doppler Effect

                   Have you ever experience something like an ambulance driving past you and noticed that the sound changes? That’s the Doppler effect in work. It applies to both sound waves and light waves. Astronomers use the Doppler effect to study the motion of objects across the Universe, including nearby extra-solar planets to the numerous distant galaxies. Doppler shift is the change in length of a wave (light, sound, etc.) due to the relative motion of source and receiver. Things moving toward you have their wavelengths shortened. Things moving away have their emitted wavelengths lengthened. For waves that multiply in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium. Each of these effects is analyzed separately. For waves which do not require a medium, such as light or gravity in general relativity, only the relative difference in velocity between the observer and the source needs to be considered.