> Which physicist discovered that a wave's frequency changes when the source andthe observer move toward or away from each other?And the answer: Christian Doppler. Photo courtesy: Soundfly.comBorn in 1803, Austrian physicist Christian Dopplerdiscovered that a light or sound wave's frequency changes when the source of thewave and its observer are moving toward or away from each other. An example ofthe Doppler effect is when an ambulance siren seems to change as it passes byyou.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 solarsystem, or "exoplanets." In fact, 442 of the 473 known exoplanets have beendetected using the Doppler effect. Light and sound frequencies are impacted by the direction that the object ismoving. For example, if a star is moving away from Earth, its spectrum of lightappears slightly redder. If it is moving toward us, its wavelength getscompressed, and the light appears more blue. Scientists use what is called aspectrograph to detect these subtle changes in color and draw conclusions aboutthe movement of faraway celestial objects. Movement of stars also informs the movement of its associated planets. A star'sDoppler shifts are the result of gravity affecting the star's motion, whichusually indicates the presence of another planet or nearby star. If the Dopplerchanges are smaller, it is more likely an object of smaller mass that isaffecting its gravity and therefore a planet. If the changes are larger, it'sanother star. Did you know?Scientists used spectrographs in the 1920s to determine that the universe wasstill 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 inferthat most stars and galaxies are moving away from us, therefore indicating thatthe universe is still growing from the Big Bang.