The Doppler Effect and Redshift: Practice Problem Solutions

1. As the ambulance approaches, the sound waves are squeezed, giving a higher pitch; as it recedes the waves are stretched, giving a lower pitch.

2. Redshift: light shifted to longer wavelengths because the source moves away. Blueshift: shorter wavelengths because it moves toward us.

3. z = (659.6 − 656.3) / 656.3 = 0.00503, so v = 0.00503 x 300,000 = about 1,500 km/s, receding.

4. It is moving toward us.

5. By the Doppler wobble of the star: a planet's gravity makes the star move slightly back and forth, which shows up as a small shift in its spectral lines (51 Pegasi b, 1995).

6. Most of them were redshifted, meaning they were moving away from us at very high speeds.

7. A Doppler redshift comes from the source moving through space; a cosmological redshift comes from the stretching of space while the light travels.