What is Doppler: Definition and 747 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. H

    Doppler effect measure speed in blood cells

    Homework Statement A Doppler blood flow unit emits ultrasound at 5.0 {\rm MHz}. What is the frequency shift of the ultrasound reflected from blood moving in an artery at a speed of 0.20 m/s? What equations would I use to calculate the doppler shift? It is not in my textbook...
  2. J

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  3. H

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  4. A

    Solve Doppler Effect Question 15: Find the Frequency Shift

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  5. A

    Doppler Effect of a security alarm

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  6. H

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  7. N

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  8. C

    Any experimental evidence of Doppler shift in light?

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  9. R

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  10. N

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  11. C

    Can the Doppler Effect Explain Redshift in Light?

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  12. M

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  13. B

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  14. A

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  15. A

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  16. J

    What Speed Is the Bat Catching Up to Its Prey?

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  17. B

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  18. G

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  19. R

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  20. A

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  21. Loren Booda

    Does Hubble's law rely on the Doppler shift?

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  22. B

    Big Bang & Doppler Effect: Viewing Start of Universe?

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  23. P

    Would Gravity Waves Show Doppler Shift?

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  24. A

    Doppler Radar Relative Velocities

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  25. F

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  26. V

    Why is the Doppler shift expression sometimes reversed?

    Hello, I am well and truly stumped on Doppler shift. The complete expression is f'=f \gamma (1-\beta cos(\theta)) where: \beta=v/c v=source\ velocity \gamma=\frac{1}{\sqrt{1-\beta^{2}}} \theta is the angle between received photons (lab frame) and the direction of motion of...
  27. Z

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  28. D

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  29. T

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  30. T

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  31. teroenza

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  32. D

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  33. T

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  34. Z

    How Does Doppler Shift Explain the Beats Heard from a Moving Car?

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  35. K

    Can Relativity Explain the Effects of High-Speed Motion on Gravity?

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  36. E

    Use the relativistic Doppler formula for light emitted

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  37. D

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  38. D

    Exploring the Doppler Effect: Understanding Pitch Changes of Police Car Sirens

    When a police car passes you with a siren with a frequency of 440 Hz, a noticeable drop in the pitch of the sound of the siren will be observed as the car passes by. But why the pitch suddenly drops when the car is passes by? I am not pretty sure why I heard the pitch suddenly drops, but I...
  39. A

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  40. M

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  41. Evo

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  42. D

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  43. P

    . Radio Spectroscopy -> Finding the doppler shift in a graph

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  44. B

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  45. B

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    Homework Statement A train is moving parallel and adjacent to a highway with a constant speed of 25 m/s. A car, at time t0, is 42 m behind the train, traveling in the same direction as the train at 49 m/s, and accelerating at 4 m/s^2. The train's whistle blows at a frequency of 460Hz. The...
  46. B

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  47. T

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  48. S

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  49. B

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  50. A

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