Electromagnetic wavesĪstronomers use the Doppler effect to measure the velocities of the galaxies with respect to the Earth. While sonar uses sound waves to measure the distance to an object, doppler sonar also measures the speed of the object. This is as if the moving object was sending out the sound waves. If a stationary source sends out sound waves and they are reflected back off a moving object, the sound waves heard at the source will be a different pitch, according to the direction of the object. Likewise, as the vehicle moves away from you, the siren would have a lower pitch. When you hear the sound of an object moving toward you-such as a siren from and emergency vehicle-the pitch of that sound is higher than if the vehicle was standing still. ( See Traveling Faster than Sound for more information.) Source is moving A speeds equal to or greater than the speed of sound, a somewhat different effect takes place. Note that the velocity of the source or moving object must be less than the speed of sound. ( See Doppler Effect for Sound for information on calculating the effect.) The Doppler effect is experienced when a source is moving and when the sound is reflected off a moving object. This is the principle used in Doppler Radar. The observed wavelength and frequency will change, according to the Doppler Effect. If both the source of the waveform and the observer are stationary but the wave reflects off a moving object, it is as if the object was a moving source. Instead, the concern is whether they are moving toward each other or away from each other. Remember that motion is relative, so we really don't care whether the source or observer is moving. Likewise, if you are moving away from the source, the observed wavelength is lengthened and the frequency is decreased. Thus the observed wavelength is shortened and the frequency is increased. If you are moving toward a stationary source, you receive the waves at a faster rate than their emitted frequency. The wavelength in the opposite direction is lower. The illustration shows how when the source of the waves moves to the right, the frequency of the waves in that direction is higher. If that source is moving with respect to a stationary observer, the speed of the waveform remains constant, but the distance between the emitted waves decreases in the direction that the source is moving and increases in the opposite direction. Note: The speed of sound can vary according to conditions, but we assume nothing is changing, so that it is constant. Thus, when λ decreases, f increases and vice-versa.
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