{"id":33854,"date":"2023-09-24T17:00:00","date_gmt":"2023-09-24T15:00:00","guid":{"rendered":"http:\/\/0a95fd17-2d9c-4ea5-9136-da3b11547f3a"},"modified":"2023-09-26T11:46:13","modified_gmt":"2023-09-26T09:46:13","slug":"why-the-humble-doppler-effect-is-the-secret-to-understanding-our-universe","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcsciencefocus\/rss_feed\/why-the-humble-doppler-effect-is-the-secret-to-understanding-our-universe\/","title":{"rendered":"Why the humble Doppler effect is the secret to understanding our Universe"},"content":{"rendered":"

This fundamental concept in physics is why the Moon sometimes looks red, and can help doctors analyse blood flow in tissues. <\/p>

By Dr Alastair Gunn\n <\/p>

Published: Sunday, 24 September 2023 at 15:00 PM<\/p>

\n\n

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\u00a0for\u00a0an observer moving relative to the source of the waves.\u00a0<\/p>

The easiest way to understand the phenomenon is to consider sound waves. Sound is characterised by the distance between successive sound waves,\u00a0its wavelength. The human ear perceives this as the \u2018pitch\u2019 of the sound. Short-wavelength sounds are higher pitched compared to sounds\u00a0with\u00a0longer wavelengths.<\/p>

Now, if the source of the sound is moving towards\u00a0you, then the waves are squashed together and their wavelength is effectively shortened, so\u00a0the pitch of the sound is higher. Conversely, if the source of the sound is moving away from\u00a0you, then the waves are stretched apart and their wavelength is lengthened\u00a0\u2013the pitch of the sound is lower.\u00a0<\/p>

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\u00a0you. You can easily hear the Dopplereffect in operation whenever a sound-emitting object is moving relative to you\u00a0\u2013 think ambulance sirens, trains, planes and so on.\u00a0<\/p>

All wave phenomena display a similar Doppler effect. In the case of light<\/a>, the wavelength emitted by an object moving towards\u00a0you\u00a0is shifted to shorter wavelengths. Since the shortest wavelengths of\u00a0visible light correspond to the perceived colour \u2018blue\u2019, this\u00a0phenomenon\u00a0is known as \u2018 blueshift<\/a>\u2019\u00a0\u2013 although it\u00a0doesn\u2019t necessarily mean the object\u00a0in question\u00a0looks blue.<\/p>

Conversely, if the source of light is moving away from\u00a0you, the wavelength is shifted towards the red end of the visible spectrum. Hence \u2018redshift\u2019, but again it doesn\u2019t necessarily mean the object\u00a0appears red.\u00a0<\/p>

The Doppler effect has\u00a0many\u00a0practical 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.<\/p>

Doppler sonars use reflected sound waves to measure the location and motion of objects. Some animals\u00a0\u2013\u00a0 bats, for example<\/a> \u2013 use a Doppler technique to detect the motion of prey, through echolocation. Doppler ultrasound is a medical imaging technique\u00a0that\u00a0uses short-wavelength sound to measure the movement of blood through the body.\u00a0<\/p>

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 \u2018spectral lines\u2019 in their observations. Chemical elements in the object emit or absorb light at specific wavelengths and act as markers in the object\u2019s light spectrum. These lines are also shifted by the object\u2019s motion relative to Earth and so can be used to measure its velocity.\u00a0<\/p>

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