This fundamental concept in physics is why the Moon sometimes looks red, and can help doctors analyse blood flow in tissues.
The Doppler effect, first described by Austrian physicist Christian Doppler in 1842, is a fundamental concept in physics. It describes the apparent change in wavelength (or frequency) of a wave phenomenon for an observer moving relative to the source of the waves.
The easiest way to understand the phenomenon is to consider sound waves. Sound is characterised by the distance between successive sound waves, its wavelength. The human ear perceives this as the ‘pitch’ of the sound. Short-wavelength sounds are higher pitched compared to sounds with longer wavelengths.
Now, if the source of the sound is moving towards you, then the waves are squashed together and their wavelength is effectively shortened, so the pitch of the sound is higher. Conversely, if the source of the sound is moving away from you, then the waves are stretched apart and their wavelength is lengthened –the pitch of the sound is lower.
This is why the sound of a racing car, for example, goes from a high pitch to a low pitch as it approaches, passes and recedes from you. You can easily hear the Dopplereffect in operation whenever a sound-emitting object is moving relative to you – think ambulance sirens, trains, planes and so on.
All wave phenomena display a similar Doppler effect. In the case of light, the wavelength emitted by an object moving towards you is shifted to shorter wavelengths. Since the shortest wavelengths of visible light correspond to the perceived colour ‘blue’, this phenomenon is known as ‘blueshift’ – although it doesn’t necessarily mean the object in question looks blue.
Conversely, if the source of light is moving away from you, the wavelength is shifted towards the red end of the visible spectrum. Hence ‘redshift’, but again it doesn’t necessarily mean the object appears red.
The Doppler effect has many practical applications. Doppler radar systems, which emit radio waves and detect the signals reflected by objects, can be used to track the movement and intensity of storms. This is also the principle on which speed cameras and air traffic control radars are based.
Doppler sonars use reflected sound waves to measure the location and motion of objects. Some animals – bats, for example – use a Doppler technique to detect the motion of prey, through echolocation. Doppler ultrasound is a medical imaging technique that uses short-wavelength sound to measure the movement of blood through the body.
The Doppler effect is crucial for astronomers. It can be used to determine the velocity (and sometimes rotation) of objects far away in space. Since all the light from a moving object is shifted by the same relative amount, astronomers make use of ‘spectral lines’ in their observations. Chemical elements in the object emit or absorb light at specific wavelengths and act as markers in the object’s light spectrum. These lines are also shifted by the object’s motion relative to Earth and so can be used to measure its velocity.
Read more:
- How does echolocation work?
- Why do bullets make a ‘zing’ sound in movies?
- Where do sound waves end up?
- Why does light travel faster than sound?
Asked by: India Kelly, via email
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