The Doppler Effect

1. The Doppler Effect

2. The Doppler Effect • Doppler Effect – Change in frequency and wavelength of a wave for an observer moving relative to the source of the wave. • Source is moving and observer is stationary. • Source is stationary and observer is moving. • Source and observer are both moving. • Named for Austrian physicist Christian Doppler. • Proposed the effect in 1842.

3. The Doppler Effect and Sound • Stationary source and stationary observer s o

4. The Doppler Effect and Sound • Moving source and stationary observer s o High frequency, short wavelength Low frequency, long wavelength

5. The Doppler Effect and Sound

6. Sonic Booms • Caused when an airplane travels faster than sound. • Supersonic = faster than sound • Sound waves “pile up” in the direction of travel. • Form a high pressure cone. • Stationary observers hear a loud boom when the front of the cone passes by.

7. Supersonic Flight A US Navy F/A-18 Super Hornet is shown here breaking the sound barrier. There is an invisible high pressure cone starting at the nose of the aircraft. The cone-shaped cloud is formed when water vapor condenses out of the air in the low-pressure wake of the high-pressure cone. This effect is called a Prandtl-Glauert singularity, or more simply, a vapor cone.

8. The Space Shuttle • Aircraft in supersonic flight actually produce 2 booms: • 1 from nose and 1 from tail. • The following video allows you to hear the twin sonic booms produced by the space shuttle Atlantis as it approaches its Florida landing strip after re-entering Earth’s atmosphere in 2008.

9. The Space Shuttle

10. The Doppler Effect and Light • Light is also subject to the Doppler effect. • Astronomers use the Doppler shift in starlight to determine if stars are moving toward or away from Earth. • Stars moving toward the Earth have their light wavelengths shortened. • Causes the light to shift toward the blue end of the visible spectrum. • Stars moving away from Earth have their light wavelengths lengthened. • Causes the light to shift toward the red end of the visible spectrum.

11. Red Shifting and Blue Shifting • The light given off by a star contains “gaps” called absorption lines. • Absorption lines correspond to the elements that make up a star. • Every element gives its own absorption signature to the star’s light. • If we know what a star is made of, we can usually predict where the absorption lines should be in the star’s spectrum of light.

12. Red Shifting and Blue Shifting • If the star is moving towards Earth, its light gets “bunched up” and shortened. • The blue-violet end of the spectrum contains the shortest wavelengths. • The incoming light from the star appears “bluer” than it would if the star were stationary relative to Earth. • All of the expected absorption lines get pushed toward the blue-violet end of the spectrum.

13. Red Shifting and Blue Shifting • If the star is moving away from Earth, its light gets “stretched out” and lengthened. • The red end of the spectrum contains the longest wavelengths. • The incoming light from the star appears “redder” than it would if the star were stationary relative to Earth. • All of the expected absorption lines get pushed toward the red end of the spectrum.

14. Red Shifting and Blue Shifting • The pair of spectra at left represent the expected absorption spectrum of a stationary star (left) and the measured spectrum of a star that is moving relative to Earth (right). • Can you tell if the star is moving toward or away from the Earth?

15. The Doppler Effect and the Big Bang • In 1912 an astronomer named Vesto Slipher measured a red shift in the light coming from a distant galaxy. • This indicated that the galaxy was moving away from Earth. • He later discovered that the light from almost all distant galaxies was red shifted. • Edwin Hubble later showed that the farther away the galaxies are, the faster they seem to be receding from us.

16. The Doppler Effect and the Big Bang • In 1927 a Belgian physicist and Roman Catholic priest named Georges Lemaître proposed that the galaxies were receding because the universe was expanding. • Four years later he would propose the first model of the Big Bang, in which the Universe expanded from an infinitely hot, dense state some time in the distant past. • Later evidence has supported this model.

17. The Big Bang Light traveling between distant galaxies gets “stretched out” (red-shifted) as the space between the galaxies expands. The individual stars in galaxies don’t move apart because they are held close together by gravity.