1. The Doppler Effect
Unit 3.5

2. The Doppler Effect
Imagine a rocket ship launched from Earth with enough fuel to allow it to accelerate to speeds approaching that of light. As the ship’s speed increased, a remarkable thing would happen.

3. The Doppler Effect
Passengers would notice that the light from the star system toward which they were traveling seemed to be getting bluer.

4. The Doppler Effect
In fact, all stars in front of the ship would appear bluer than normal, and the greater the ship’s speed, the greater the color change would be.

5. The Doppler Effect
Furthermore, stars behind the vessel would seem redder than normal, while stars to either side would be unchanged in appearance.

6. The Doppler Effect
As the spacecraft slowed down and came to a rest relative to Earth, all stars would resume their usual appearance.

7. The Doppler Effect
The travelers would have to conclude that the stars had changed their colors not because of any real change in their physical properties, but because of the spacecraft’s own motion.

8. The Doppler Effect
This phenomenon is not restricted to electromagnetic radiation and fast-moving spacecraft.

9. The Doppler Effect
Waiting at a railroad crossing for an express train to pass, most of us have had the experience of hearing the pitch of a train whistle change from high shrill to a low blare.

10. The Doppler Effect
This motion-induced change in the observed frequency of a wave is known as the Doppler effect.

11. The Doppler Effect
Applied to cosmic sources of electromagnetic radiation, it has become one of the most important measurement techniques in all of modern astronomy.

12. The Doppler Effect
Imagine a wave moving from the place where it is created toward an observer who is not moving with respect to the source of a wave.

14. The Doppler Effect
By noting the distance between successive crests, the observer can determine the wavelength of the emitted wave.

15. The Doppler Effect
Now suppose that not just the wave, but the source of the wave, also is moving.

16. The Doppler Effect
Because the source moves between the times of emission of one crest and the next, successive crests in the direction of motion of the source will be seen closer together than normal, whereas crests behind the source will be more widely spaced.

17. The Doppler Effect
An observer in front of the source will therefore measure a shorter wavelength than normal, while one behind will see a longer wavelength.

18. The Doppler Effect
The greater the relative speed between source and observer, the greater the observed shift.

19. The Doppler Effect
A wave measured by an observer situated in front of a moving source is said to be blueshifted, because blue light has a shorter wavelength than the red light.

20. The Doppler Effect
Similarly, an observer situated behind the source will measure a longer-than-normal wavelength-radiation is said to be redshifted.

21. The Doppler Effect
Because the speed of light is so large, the Doppler effect is extremely small for everyday terrestrial velocities.

22. The Doppler Effect
The importance of the Doppler effect to astronomers is that it allows them to find the speed of any cosmic object along the line of sight simply by measuring the extent to which its light is redshifted or blueshifted.

23. The Doppler Effect
The motions of nearby stars and distant galaxies – even the expansion of the universe itself – have all been measured in this way.