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cscott
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Why is it that the extent of the doppler effect on sound depends on whether, for example, you are moving towards the source or the source is moving towards you? Why does this not happen for light?
The precise reckoning of the doppler effect was a matter of great importance to Einstein, and he found that light did not behave in exactly the same way as sound. Because sound waves travel throught a medium - the air - the extent of the doppler shift depends on whether the source of the sound is moving towards the listener or the listener is moving towards the source of sound.
[...] In Einstein's democratic universe, that cannot make any difference: all that matters is he relative speed of th start and the onlooker. The correction is a small but critical one - critical because blue light is inherently more energentic that red light, so that changing the colour of light affects its energy. [...]
Sound travels about 350 metres per second. A train traveling at 120 kilometres per hour is moving 33 1/3 metres per second, or roughly one tenth the speed of sound. From Doppler's formula, we can expect the frequency of sound approaching the train to be shifted to 10% higher pitch, and sounds receding shifted to 10% lower pitch, roughly the difference between two adjacent white keys on a piano keyboard. If a pipe organ in a church near the train tracks is playing this note, a passenger on a (very quiet Swiss) train approaching the church at 120 km/hour will hear the note shifted upward by about 10% in frequency. After the train passes the church, the note will be heard shifted down by the same percentage.
cscott said:Why is it that the extent of the doppler effect on sound depends on whether, for example, you are moving towards the source or the source is moving towards you? Why does this not happen for light?
Meir Achuz said:"The precise reckoning of the doppler effect was a matter of great importance to Einstein, and he found that light did not behave in exactly the same way as sound. Because sound waves travel throught a medium - the air - the extent of the doppler shift depends on whether the source of the sound is moving towards the listener or the listener is moving towards the source of sound.
[...] In Einstein's democratic universe, that cannot make any difference: all that matters is he relative speed of the start and the onlooker."
These two quotes state the correct situation.
The details of the difference depend on the different derivations
(and can be seen in the formula for each case), but the basic difference is that there is a medium for air, and not for light.
I will have to give the relativistic formula for the Doppler shift:rbj said:if the author means "velocity" (as a vector) instead of speed, then i agree. but the doppler effect on light coming from a source moving toward an observer will be different than the doppler effect from the same source moving away from the observer at the same speed.
Meir Achuz said:I will have to give the relativistic formula for the Doppler shift:
w'=w gamma[1+(v/c) cosA], where v is the speed of the star and A is the angle between the star's velocity and the line from the star to you, all in your rest system. The formula is the same whether you or the star is moving, but the light is always observed by you in your rest system.
The Doppler effect of sound does depend on who is moving. If I were to move backwards at a speed a little greater than the speed of sound from a speaker I would out run the sound and never hear it. But if I moved the speaker back and the same speed i would eventually hear the sound.russ_watters said:I don't think that first paragraph is correct. A quick google shows that the doppler shift equation for sound doesn't differentiate who is really moving.
The Doppler Effect is a phenomenon that occurs when there is a change in frequency of a wave due to the relative motion between the source of the wave and the observer.
The Doppler Effect applies to both sound and light waves. When the source and observer are moving closer together, the frequency of the wave increases, resulting in a higher pitch or blue shift for sound and a shorter wavelength or higher energy for light. When the source and observer are moving farther apart, the frequency of the wave decreases, resulting in a lower pitch or red shift for sound and a longer wavelength or lower energy for light.
Yes, the Doppler Effect can be observed in everyday life. For example, when an ambulance or a police car passes by with its siren on, you may notice a change in pitch as it approaches and then passes you. This is due to the Doppler Effect. You can also observe the Doppler Effect with light, such as when a car with its headlights on passes by you at night and the light appears to change color as it approaches and then passes by.
The Doppler Effect is used in various scientific and technological fields. In astronomy, it is used to measure the relative motion of stars and galaxies. In medicine, it is used in ultrasound technology to detect and measure blood flow. In weather forecasting, it is used to track the movement of storms. It is also used in police radar and in satellite communication systems.
Yes, the speed of the source and the observer affect the Doppler Effect. The greater the speed of the source or the observer, the greater the change in frequency or wavelength. This is because the relative motion between the source and the observer is a key factor in the Doppler Effect.