What is the Doppler Effect?

eric

Stand on the sidewalk and listen to the sound of a car as it approaches and passes by. You will notice that the pitch of the sound is higher as the car approaches and then becomes lower as it moves away. That change in pitch is the result of the Doppler Effect.

How does it happen? Sound waves move outward from the source of the sound though the air in all directions. The pitch of the sound results from the spacing in the sound waves. If the waves are closer together, the sound has a higher pitch. If they are farther apart, a lower pitch. This spacing between waves, or the distance from the crest of one wave to the next, is called the wavelength.

Let’s look at two different situations. The first diagram below represents a car that is stopped at a light. We’ll concentrate on a single sound, perhaps the hum of the engine. Each circle represents a crest of the sound wave moving outward.

Sound waves moving outward from a stationary car

Sound waves moving outward from a stationary car

If we were to examine the same car a little later, each circle would be bigger, but distance between each circle would remain the same. As the wavelength of the sound is the same in all directions, anyone that is stationary relative to the car will hear the same pitch.

In the second diagram below, the car is moving to the right. Because the source of the sound wave (the car) moves between the times when two wave crests leave the source, the wave crests end up closer together in the direction of motion and farther apart in the opposite direction.

Sound waves moving outward from a moving car

Sound waves moving outward from a moving car

So, someone standing at point A will hear a higher pitch and someone standing at point B will hear a lower pitch. Since the pitch that they hear depends on the car’s speed, they could find out how fast the car is moving by measuring the shift in the sound’s wavelength.

The Doppler Effect also works for light. In the case of light, different wavelengths of light are different colors. Blue light has short wavelengths and red light has long wavelengths. If an object is moving toward us, particular colors of light given off by that object have a shorter wavelength than they do when stationary and we say the light is “blue shifted.” Likewise, if an object is moving away from us, particular colors of light given off by the object have a longer wavelength than they do when stationary and we say the light is “red shifted.” By carefully measuring the apparent shift in the wavelength of the light, astronomers can determine how fast an object is moving toward or away from us.

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8 thoughts on “What is the Doppler Effect?

  1. thaks very helpfull i’m a sudent of architecture my project deals with acoustics on higways and car noise ,and would like to know how the sound waves look like from the side view not only from the top please if you can help me i’ll be very glad
    thanks lena

  2. Lena,
    In still air, sound waves from an object are spherically symmetric. For sound waves above the ground, the side view will be the same as the top view. This simplified picture only applies to sound waves from the object making the sound. It does not include sound waves reflected by the ground or other objects. Reflected sound waves can add to or subtract from the original sound wave. This combining of sound waves is called interference. Interference can complicate the picture significantly.

  3. Hi there

    I’ve been really curious about something that I heard on the news when the USA went into Pakistan to get Osama Bin Laden.

    I heard that the fancy helicopter that crashed was fitted with technology that made it sound like it was getting further away even though it was getting closer.

    My first reaction was that surely this is impossible, because of the doppler effect.

    Do you have any thoughts on that? It’s got to be rubbish surely!

    Cheers
    Leigh

  4. Leigh,

    By their very nature, one half of a helicopter’s rotating blades will look like they’re coming faster towards you than the helicopter, and one half will look like they’re going away from you faster than the helicopter. That’s just the nature of the rotating blades.

    I don’t know much about stealth helicopters. I suppose that if the fuselage was really “stealthy” and didn’t have much of a return signal on radar then you might be left with just the signal from the main roter blades, and that might give you some kind of a weird, “it’s coming _and_ it’s going,” doppler return. Just speculating.

    Stars can show a similarly confusing doppler shift because they’re being tugged back and forth by the gravity of planets in orbit around them. In fact, looking for stars with spectra that appear to be shifted both towards the red and the blue (the star looks like it’s simultaneously approaching and receding) is one of the most commonly used ways astronomers use to detect exoplanets.

  5. I’m curious, do other types of waves besides sound/light experience the doppler effect? If so, how does it manifest itself, e.g. radio?

  6. Hello.
    In diagnostic ultrasonography what kind of waves travels through tissue or media?
    1.sound waves or
    2.transverse waves or
    3.pulsed waves or
    combination of waves.

    Could you explain,please.

  7. Benedict,

    Visible light is one form of something that we call electromagnetic radiation. Radio, microwaves, infrared, ultraviolet, x-rays, and gamma rays are other forms of electromagnetic radiation. They are all exactly the same as visible light except that they have different wavelengths. That is the only difference. (The fact that human eyes can only see a narrow range of these wavelengths does not make them different). Therefore, all electromagnetic radiation experiences the Doppler Effect. It is manifested as a shift in the wavelength, just like visible light.

  8. Natasha,

    Diagnostic ultrasound uses high frequency (typically 1-20 MHz) sound waves. Pulses of high frequency sound waves are generated by an ultrasonic transducer that is placed on the patient’s skin. While most of the sound waves are scattered and absorbed within the patient, some are reflected by internal body structures (at boundaries between two dissimilar materials). When these reflected sound pulses return to the surface, they are received by the transducer and then amplified and processed to form an image.

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