What is Doppler shift: Definition and 148 Discussions

The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842.
A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.The reason for the Doppler effect is that when the source of the waves is moving towards the observer, each successive wave crest is emitted from a position closer to the observer than the crest of the previous wave. Therefore, each wave takes slightly less time to reach the observer than the previous wave. Hence, the time between the arrivals of successive wave crests at the observer is reduced, causing an increase in the frequency. While they are traveling, the distance between successive wave fronts is reduced, so the waves "bunch together". Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency. The distance between successive wave fronts is then increased, so the waves "spread out".
For waves that propagate 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 electromagnetic waves or gravitational waves, only the relative difference in velocity between the observer and the source needs to be considered, giving rise to the relativistic Doppler effect.

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  1. sweetdreams12

    Why isn't the light signal from a falling spaceship Doppler shifted?

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  2. J

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  3. R

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  4. U

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  5. B

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  6. Z

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  7. B

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  8. S

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  9. P

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  10. F

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  11. skate_nerd

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  12. L

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  13. D

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  14. G

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  15. Solarmew

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  16. R

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  17. B

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  18. A

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  19. N

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  20. A

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  21. G

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  22. Daaavde

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  23. M

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  24. N

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  25. H

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  26. M

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  27. R

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  28. Y

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  29. C

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  30. L

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  31. A

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  32. Y

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    I have re-written this as as I accidently deleted my original post. I was wondering if the relativistic Doppler shift of a reflection from a mirror moving away from the observer was the same as the Newtonian equation in the special case that the mirror is orthogonal to the direction of motion...
  33. S

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  34. C

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  35. C

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  36. T

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    Hi, I'm trying to understand a derivation / question for my revision. It has to do with the four momentum of a photon and doppler/redshifts. Homework Statement Show that ratio between emitted and received frequencies (for two observers E and R moving with velocities u and v respectively)...
  37. V

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    Hi! I have two (stupid) questions: 1) We can determine what elements are present in distant stars by studying their spectrum, right? But if the spectrum of some star is redshifted, wouldn't we be fooled into believing some elements to be present there that might not really be there, due to...
  38. A

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  39. T

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  40. M

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  41. E

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  42. I

    Understanding Doppler Shift Equations for Calculating Object Velocity

    So I'm planning to start a project using the Doppler shift to calculate the velocity of a moving object. The problem is, I'm not sure which equations I should use, and I saw many different equations around the Internet. So are these equations correct? If not, can someone tell me what the...
  43. M

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    I was talking to one of my friends the other day about doppler effect and we went into the discussion of accelerating object and doppler effect. we both agreed that the frequency shift would vary in time. then we argued about the frequency shift for different observers and here is were it got...
  44. bcrowell

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  45. B

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  46. B

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  47. R

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  48. A

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  49. P

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  50. J

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