The Doppler Effect

Which physicist discovered that a wave's frequency changes when the source and the observer move toward or away from each other?

And the answer: Christian Doppler.

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Born in 1803, Austrian physicist Christian Doppler discovered that a light or sound wave's frequency changes when the source of the wave and its observer are moving toward or away from each other. An example of the Doppler effect is when an ambulance siren seems to change as it passes by you.

While we’re more aware of the Doppler effect as it screams by us on the street, it is also a tool used by astronomers to detect planets outside of our solar system, or "exoplanets." In fact, 442 of the 473 known exoplanets have been detected using the Doppler effect.

Light and sound frequencies are impacted by the direction that the object is moving. For example, if a star is moving away from Earth, its spectrum of light appears slightly redder. If it is moving toward us, its wavelength gets compressed, and the light appears more blue. Scientists use what is called a spectrograph to detect these subtle changes in color and draw conclusions about the movement of faraway celestial objects.

Movement of stars also informs the movement of its associated planets. A star's Doppler shifts are the result of gravity affecting the star's motion, which usually indicates the presence of another planet or nearby star. If the Doppler changes are smaller, it is more likely an object of smaller mass that is affecting its gravity and therefore a planet. If the changes are larger, it's another star.

Did you know?

Scientists used spectrographs in the 1920s to determine that the universe was still expanding. By measuring the extent to which their light is "stretched" into the lower frequency, or red part of the spectrum, they were able to infer that most stars and galaxies are moving away from us, therefore indicating that the universe is still growing from the Big Bang.

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