The Doppler effect, also known as the Doppler shift, is the change in frequency of a wave for an observer moving relative to its source. The Austrian physicist Christian Doppler (1803—1853) proposed this effect in 1842 in Prague. This effect is very commonly heard in the sounding siren of an emergency vehicle as it approaches, passes, and then gets farther away from another vehicle or observer. The received frequency is always higher than the emitted frequency during the approach of the observer, while the frequency is identical at the direct instant of passing the observer. As the source gets farther away from the observer, the received frequency becomes lower than the emitted frequency.
While the concept was first proposed by Christian Doppler in 1842, Christophorus Henricus Diedericus Buys Ballot (1817—1890), a Dutch chemist and meteorologist, tested the Doppler effect hypothesis for sound waves in 1845. His test confirmed that the sound’s pitch was higher than the emitted frequency when the sound approached him, and lower than the emitted frequency when the sound receded away from him. French physicist Hippolyte Fizeau (1819—1896) independently discovered the same phenomenon in 1842 with electromagnetic waves; and in 1848, Scottish civil engineer John Scott Russell (1808—1882) made an experimental study of the Doppler effect in Britain.
Where Do We See the Doppler Effect?
The Doppler effect is seen almost everywhere we see waves. As already mentioned, humans can hear the Doppler effect in sirens or other loud passing sounds such as airplanes flying overhead. It can also be observed within electromagnetic waves and light waves.
In astronomy, the Doppler effect is observed in electromagnetic waves. Light is of great use in astronomy and can result in what is called redshift or blueshift depending on the motion of the object creating the light. The Doppler effect in astronomy is used to measure the speed at which stars and galaxies are approaching or receding from Earth (radial velocity). The spectra of stars exhibits absorption lines at well-defined frequencies that are correlated with the energies required to excite electrons in various elements from one level to another. These absorption lines are not always observed at the frequencies that are expected from the spectrum of a stationary light source. Blue light has a higher frequency than red light, and therefore the spectral lines that are an approaching astronomical light source exhibit a blueshift and those of a receding astronomical light source exhibit a redshift.
Radar uses the Doppler effect to measure the velocity of various detected objects. A radar beam is fired at a moving target and is continually fired as the target moves farther away or closer to the source. The radar beam determines if the gap is getting bigger or smaller between firing, and the wavelength increases or decreases to compensate. If the beam travels a shorter distance, a smaller wavelength is used, and vice versa. This allows the location and velocity of various objects to be measured from a still location. Today, the weather is predicted using the Doppler effect to track storms and changes in air patterns.
In medicine, the Doppler effect is used in medical imaging and in measuring blood flow using tools such as ultrasound or echocardiogram (ECG). An ECG can produce an accurate assessment of the direction of blood flow and the velocity of blood and cardiac tissue at any arbitrary point using the Doppler effect. Although an ultrasound beam functions similarly, it unfortunately must be parallel to the blood flow in order to provide an accurate reading of the velocity. Through calculations using the Doppler effect, cardiac output can be determined.
A group at Microsoft as well as a group at the University of Washington have performed research on using the Doppler effect to sense gestures while interacting with a computer. They used sonic techniques (Doppler effect) to test for gesturing (motion sensing). Some gestures they tested for include but are not limited to scrolling, single tap or double tap, and the two-handed seesaw. They used software called SoundWave to build a computer that is only capable of motion sensing via the Doppler effect. While this research is still in process, they believe that this software could be implemented among many different devices including cellphones and tablets. If this is the case, it might provide the opportunity for cheaper tools to be used in the medical world. This could have huge implications in developing countries and underserved areas where the finances for equipment are limited.
See also: Auditory System; Visual System
Gupta, Sidhant, Dan Morris, Shwetak N. Patel, & Desney Tan. (2012). SoundWave: Using the Doppler effect to sense gestures. ACM SIGCHI Conference on Human Factors in Computing Systems, May 5—10, Austin, Texas.
National Aeronautics and Space Administration (NASA). (2015). Doppler effect. Edited by Nancy Hall. Retrieved from https://www.grc.nasa.gov/www/k-12/airplane/doppler.html