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.

View More On Wikipedia.org
Recent contents Top users
  1. R

    Rearranging equation - Relativistic doppler

    Homework Statement The hydra cluster is 900 Mpc (3x10^9 light years) away. Calcium atoms emit a spectral line of wavelength 396.8 nm. This line is observed at 487.5 nm. The velocity of the cluster, v, is given by (\lambda =c/f) 487.5 = 396.8 SQRT((1+v/c)/(1-v/c)) How do i find v please...
  2. A

    Doppler Effect radio broadcasting

    Homework Statement Three radio-equipped plumbing vans are broadcasting on the same frequency f_0. Van 1 is moving north of van2 with a speed of v, van 2 is fixed, and van 3 is moving west of van 2 with a speed of v. What change in frequency (f-f_0) does van 3 hear from van 1? Homework...
  3. O

    The Doppler Effect of radar gun

    Homework Statement You are in charge of calibrating the radar guns for a local police department. One such device emits microwaves at a frequency of 2.15 GHz. During the trials, these waves are reflected from a car moving directly away from the stationary emitter. You detect a frequency...
  4. H

    How Does Source Motion Affect Frequency in the Doppler Effect for Sound?

    we know...when a source is running towards the listener...the wavelenth of sound become small and number of waves inceases...as far as i know ..the logic behind is..when a source runs..then waves become congested in a small distance between source and listener...it is so far so ridiculous to me...
  5. S

    Absolute velocity, CMB, and doppler shift

    I've heard it said that it is impossible to determine an objects absolute velocity, all one can do is find it relative to some other object... but, if the CMB is everywhere, why could you not just measure the doppler shift in all directions, and adjust your velocity so the CMB has no doppler...
  6. I

    Derivation of Doppler Shift from Frequency to Wavelenght

    Homework Statement Derive the doppler shift equation from the equation in terms of frequency to one in terms of wavelenght. Clues: frequency=c/lamda, use Taylor's expansion, velocity of source is much smaller than velocity of wave. c-velocity of wave v-velocity of source f-frequency...
  7. ideasrule

    The Flyby Anomaly: Ranging Data, Transverse Doppler Effect

    One paper claims that the flyby anomaly only exists because NASA scientists forgot about the transverse Doppler effect. If that's true, why is it still called an anomaly? Why isn't everyone banging their heads on walls and thinking, "D'oh! I can't believe I forgot about that!"? If it's not...
  8. A

    Doppler Effect: How Does the Ambulance Siren Vary?

    When an ambulance approaches, its siren sounds shriller than when I relatively motionless to it, and vice versa. Such is an example of doppler effect, isn't it? However, there one thing I am not sure: does the siren gradually becomes more shriller when the ambulance approaches, or not...
  9. M

    How to Calculate Buoyant Force and Volume of a Submerged Object?

    doppler effect quick question! Homework Statement An ambulance traveling at speed 31 m/s is producing a loud sound of frequency 895 Hz. You see the ambulance in your rear view mirror as the ambulance approaches you and you slow down to 14 m/s as a law obeying citizen (hehehe we know now)...
  10. S

    Doppler effect of an ambulance

    Homework Statement An ambulance is running on an expressway at a speed of 60 km/h from east to west (from A to B) with a siren of 880 Hz. Let the frequency of the siren sound detected by an observer located at a point O be Va and Vb when the vehicle just passes point A and point B respectively...
  11. P

    Energy loss in the doppler effect?

    If the energy of an EM wave = h x f and when a light emitting body (of a particular power) is moving toward or away from the observer the observed frequency changes, does the energy change? If so where does the gained or lost energy come from?
  12. U

    The Doppler Effect (in general)

    I have a problem in regard to the doppler effect,which may be generalised to all waves--sound,water etc. Please explain why the observed frequency increases as the object approaches an observer and then decreases as the object passes the observer.Actually,I thought that since doppler effect...
  13. N

    SR and classical Doppler shift

    Alice and Bob are each in their own spaceship, and are traveling radially, initially away from each other, and subsequently toward each other, at some highly relativistic relative speed, say 99c. Each ship emits light flashes toward the other. Alice observes the light from Bob's outbound...
  14. P

    Doppler Effect: Speed of Sound and Wind Question

    Hi, I've got a problem that I can't seem to find an explanation to, about the Doppler Effect An ambulance is going down the road at a speed U meters/second, in the same direction as the wind, which has a speed of W meters/second. The speed of sound in air is c meters/second Now, I have to...
  15. E

    Problem with Doppler Broadening

    Hello, I would like to know how to calculate the broadening of the spectral lines caused by the Doppler effect for the Lyman, Balmer and Paschen series. To be more concrete, I would like to know the broadening of the alpha transitions. The equations I use are the following but i don't know if...
  16. M

    How Do You Calculate the Maximum Speed of a Speaker in Simple Harmonic Motion?

    Homework Statement A loud speaker is connected to a string (spring constant k = N/m). The speaker moves back and forward in simple harmonic motion with an Amplitude of 0.8m. The frequency of an oscillation for amass on a string is w = (k/m)^1/2. The mass of the speaker is 90.0g. What is...
  17. W

    Calculating Bat Flight Speed Using the Doppler Effect

    Homework Statement A bat flies toward a wall, emitting a steady sound with a frequency of 25.0 kHz. This bat hears its own sound plus the sound reflected by the wall. How fast should the bat fly, v_b, to hear a beat frequency of 220 Hz? Take the speed of sound to be 344 m/s...
  18. M

    Doppler Effect on Light: Understanding Red and Blue Shifts in Galaxies

    Hi, I'm new to these forums. I just had a question about the Doppler effect on light (red and blue shifts of light). I've read how it is supposed to work, but I also read in a book that the speed of light is constant in all moving frames. So if you've got a galaxy moving towards you, and...
  19. T

    Doppler effect from an angle

    Doppler Effect- Observer On Angle To Source Plz help- I've been working on this for 2 days and I'm down to my last teaspoon of coffee powder. Even having the answer didn't help! I think sleepness is turning me into a zombie :eek: ...prehaps eating brains will help me solve this problem...
  20. A

    Maxwells equation and Doppler Shift.

    I am trying to grasp how Maxwell's equations and energy/photons are related in a doppler shift problem. The way I stated it seems to have attracted non-serious hecklers; so I am trying to restate the problem for people who know Maxwell's equations and are familiar with engineering. In...
  21. A

    The doppler radar trap paradox on the path to gravity.

    Hello All, I have been challenged by a friend to look into Gravitational waves and some questions he has about them -- but I have always been a slow starter. Took physics with SR in college, got my BSEE, happily can build analog circuitry of all kinds -- but found that certain questions about...
  22. S

    What is the distance traveled by a rollercoaster based on the Doppler effect?

    Homework Statement Standing in line next to the scariest rollercoaster of all time, you see your friends screaming their heads off as they come speeding out of the end of the ride. The sound of their screams is about 30% higher in pitch than normal due to the Doppler shift caused by their...
  23. L

    How Do You Calculate Wavelength in Doppler Effect Scenarios?

    Homework Statement source emits sound with a characteristic frequency of 640 Hz. Calculate the observed frequency and the wavelength between source and observer in each of the following situations. a) The observer approaches a stationary source at 57 m/s b) Both the source and observer move...
  24. L

    Doppler Effect of a blown whistle

    Homework Statement A whistle of frequency 500 Hz tied to the end of a string of length 1.2 m revolves at 400 rev/min. A listener standing some distance away in the plane of rotation hears frequencies in the range ____? (speed of sound=340 m/s) a) 436 to 586 b) 426 to 574 c) 426 to 584 d)...
  25. L

    Calculating Beat Frequency in the Doppler Effect

    Homework Statement A boy is walking away from a wall towards an observer at a speed of 1 m/s and blows a whistle whose frequency is 680 Hz. THe number of beats heard by the observer is ____. (Velocity of sound in air=340 m/s) a) zero b)2 c)8 d)4 Homework Equations The Attempt at...
  26. K

    Does the observed frequency approach infinity or does it become negative?

    I chose to write an essay on the "interesting" question I stumbled upon For part of it I am trying to calculate the Doppler shift in light with the relative motion of C. I suppose in the effort to not drone on for days I will first limit it to small, more simple questions and later,if we...
  27. A

    Doppler Effect frequencies Question

    Homework Statement An object producing a sound with frequency 500 Hz is traveling at 40 m/s toward an observer, and the observer is traveling at 30 m/s away from the source. Another source producing a sound with the same frequency is traveling at 10 m/s toward a stationary observer...
  28. C

    Doppler Shift for Traffic Light Colors

    Homework Statement How fast would a motorist have to be traveling for a yellow (l = 595 nm) traffic light to appear green (l = 550 nm) because of the Doppler shift? Homework Equations v=[(c)(f0/fs)2-c] / [(f0/fs)2+1] The Attempt at a Solution v= Speed of motorist c= Speed of light...
  29. L

    Doppler effect for light wave

    How to prove the doppler effect for light wave? Isn't that the speed of light is constant for both the observer and source? If the wavelength for both the source and observer is the same, then the frequency is the same too, so there is no doppler effect? If the wavelength is different...
  30. E

    Doppler effect and expanding universe

    Astronomers believe that the universe is expanding because of the measured doppler effect redshift of light emitted from distant stars in other galaxies, correct? Do they have other reasons for believing that the universe is expanding? The stars in our own galaxy are known to not be traveling...
  31. P

    Basic problem regarding doppler effect of light (E/M waves)

    "Basic" problem regarding doppler effect of light (E/M waves) 1. At what speed would one have to move at to see a red light as green. Presume red light wavelength = 630nm and green light wavelength = 530nm. 2. f0=fs√([1+v/c]/[1-v/c]) >f0 = observed frequency >fs = source frequency >v...
  32. K

    The doppler effect on light (red and blue shifts)

    what does the doppler effect mean for an individual photon? for instance in a blue shift do the photons actually have more energy if they were originally a lower frequency?
  33. C

    Understanding the Doppler Effect of Light: How Speed Impacts Color Perception

    I remember reading in a book that if we approached a red light really fast it would appear green to us due to the Doppler effect of light does anyone know how fast we would need to travel for this to happen.
  34. B

    Are the formulas that account for aberraion and doppler shift compatible?

    Presenting the aberration of light effect we consider that two observers at rest in I and I' in the standard configuration equipped with laser guns start to emit light when they are instantly located at the same point in space. The problem is to find out a relationsship between the directions...
  35. I

    What is the Doppler Effect of a Car's Music at Different Positions?

    Homework Statement A car is driving down the road at 25m/s. Inside the car, the radio is blaring music at 1800Hz. How would it sound to a person standing still a) in front of the car b) behind the car?Homework Equations [fo/(vw+vo)] = [fs/(vw-vs)]The Attempt at a Solution a) [fo/(vw+vo)] =...
  36. I

    What happens when a wave's wavelength is compressed by moving objects?

    Homework Statement A radar beam has a wavelength of .02 m. The frequency is 17500 Hz. What would be its echo frequency if a rocket 60km away is moving at 500m/s and the radar is coming from a jet moving at 310m/s towards the rocket which is aimed at it? (b) Would the rocket appear red shifted...
  37. J

    Conceptual Doppler Effect question

    Homework Statement A train approaches, and passes through, a station. During this period the velocity of the train is constant and the engine is continuously sounding its whistle. Which one of the following correctly describes what an oberver on the platform will hear? Sound heard as...
  38. S

    Doppler Effect ambulance speed Question

    Homework Statement An ambulance with a siren emitting a whine at 1530 Hz overtakes and passes a cyclist pedaling a bike at 2.22 m/s. After being passed, the cyclist hears a frequency of 1519 Hz. How fast is the ambulance moving? (Take the speed of sound in air to be 343 m/s.) Homework...
  39. M

    What is the solution to the Doppler Effect and Fire Truck Homework?

    Homework Statement Hearing the siren of an approaching fire truck, you pull over to the side of the road and stop. As the truck approaches, you hear a tone of 470 Hz; as the truck recedes, you hear a tone of 400 Hz. How much time will it take the truck to get to the fire 2.5 km away assuming...
  40. D

    "Doppler Effect: Applying a 35 m/s Car

    Homework Statement A car moving at 35 m/s approaches a stationary whistle that emits a 220 Hz sound. The speed of sound is 343 m/s. What is the speed of sound relative to the driver of the car? Homework Equations fo = fs [1 +/- vo/v] / [1 +/- vs/v] The Attempt at a Solution Dont...
  41. D

    Doppler Effect & Beat Frequency

    Homework Statement Horseshoe bats use the Doppler Effect to determine their location. A horseshoe bat flies toward a wall at a speed of 15 m/s while emitting a sound of frequency 35 kHz. What is the beat frequency between the emission frequency and the echo? Assume that the air temperature is...
  42. W

    Apparent Frequency (Doppler) Problem

    The security alarm on a parked car goes off and produces a frequency of 952 Hz. The speed of sound is 343 m/s. As you drive toward this parked car, pass it, and drive away, you observe the frequency to change by 97 Hz. At what speed are you driving? F=Fo [(Vsound-...
  43. L

    Thim experiment: Doppler expected or not?

    Hello, This paper presents an experimental setup and claims that Special Relativity would predict a second order Doppler shift: http://www.atomicprecision.com/blog/wp-filez/Thim%20-%20Absence%20of%20the%20relativistic%20Doppler%20effect%20...%20.pdf Since the emitter and receiver have...
  44. B

    Validity of the Doppler shift formula?

    The formula which accounts for the Doppler shift is derived from the invarainace of the phase of a plane wave. [1] The plane wave is a mathematical construction associated with an infinite source os oscillation or with a very high source-receiver distance. The same formula is derived...
  45. K

    The Doppler Effect and Redshifting ?

    If I could please get some help on these really puzzling questions, that would be great. Consider these formulas: If a light source with frequency fo is moving away at speed v, then you see the frequency to be lower according to f=fo( (the square root of 1-(v/c)) / (the square root of...
  46. A

    Finding the Right Book to Understand the Doppler Effect

    hi friends which is nice book for explanation of doppler effect please help me
  47. H

    Energy conservation for a relativistic doppler shifted pulse

    I am new to the field of relativity. I read the Lorentz transformation between different system of coordinates. I have a question. Let's suppose that we have a Doppler shifted pulse in frequency (time dilation). This pulse has the same amplitude as the pulse seen by a moving observer. That is...
  48. J

    Doppler Effect: Sound Level at 40m from Source of 20dB

    At a distance of 20m from a source of sound, the sound level is 40dB. If the observer backs up to a distance of 40m from the source, what will the sound level be? Basically, I said that the sound level halved = 20dB. Correct?
  49. J

    Understanding the Effects of Doppler Shift on Sound Perception

    You are standing on railroad tracks as a train approaches at a constant velocity. Suddenly the engineer sees you, applies the brakes, and sounds the whistle. What do you hear starting at that moment? Because the waves are "stretching" as it approaches you slower and slower, I said that the...
  50. H

    The Doppler Effect and Simple Harmonic Motion question

    Homework Statement Ok here's the problem.. a little lengthy so bear with me :) (3 parts need help w/ last two) Expectant parents are thrilled to hear their unborn baby's heartbeat, revealed by an ultrasonic motion detector. Suppose the fetus's ventricular wall moves in simple harmonic...
Back
Top