The Doppler Effect

What is the Doppler Effect? The shift in frequency and wavelength of a wave There are three reasons for this occurance: 1) The source of the wave moves 2) The receiver moves 3) The medium itself moves

Stationary Wave Source Waves produced at a constant frequency & wavelength move at a constant velocity Waves propagate symmetrically from source Everyone observes same frequency

Doppler Shift Source Moving to Right Because the source is moving to the right, the center of each new wave is displaced to the right This causes the waves to bunch up on the right, and spread out on the left Right: Greater frequency, lesser wavelength Left: Lesser frequency, greater wavelength

Interactive Hyperlink Examples Police Car Waves Police Car Sound

Doppler Shift Sound Examples Helicopter Train 1 Train 2

Train Example No Movement: Movement:

Astronomical Red & Blue Shift Just as the wavelength and frequency of sound waves change with motion, so do light waves If a source moves to the right, the wavelength of the light waves on the right get smaller, and the waves on the left get bigger This causes shifts on the light spectrum

Cont’d When the wavelength of light waves shortens, the light moves toward the blue end of the light spectrum (blue shift) When the wavelength of light waves gets larger, the light moves toward the red end of the light spectrum (red shift) Astronomers use Doppler shifts to calculate precisely how fast stars and other astronomical objects move toward or away from Earth.

How does this relate to the Origin of the Universe? We know that Stars are made of atoms (mostly hydrogen and helium) which emit specific frequencies of light. In the 1930’s Edwin Hubble noticed that the spectra of most stars (and galaxies) were Red Shifted. These stars must be moving away from us Scientists have concluded that if run this motion in reverse all the stars would start from the same point. This is evidence for the Big Bang Theory

Stop Here

History Doppler Effect developed by Austrian physicist Christian Doppler ( ) Proposed the “Doppler Effect” in 1842 Conducted a very unique experiment to test his theory For two days, Doppler had a train pull repeatedly at different speeds a freight car with trumpeters playing on top of it.

Cont’d He then had a musician capable of understanding the differences in sounds record the height of the notes played as the train moved closer or further away. This helped support his theory greatly He then wanted to show that his theory also applied to light, but never got to; this was later supported by another scientist named Fizeau The Doppler Effect has had an important influence in astronomical studies and understanding the universe and it’s properties

Christian Doppler ( )

Applications of the Doppler Effect Many uses in science and practical applications as well Measurements of shifts of radio waves from orbiting satellites Used in maritime navigation Navigational system called Transit NAVSTAR, part of GPS

Cont’d Employed in weather forecasting Next-Generation Radar (NEXRAD) digital weather radar Radar detection of dangerous winds for aircraft Radar surveillance of automobile speeds Moving-target indication (MTI) by use of Doppler shift in frequency that radio wave undergoes when reflected from a moving target

Cont’d Medically, Doppler effect used in ultrasonography (study of motions in deep-lying body structures) Also used in echocardiography (study of heart motions)