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A hydrogen line normally at 500 nanometers is measured at 500.1 nanometers in a star's spectrum. What does that tell you?
AIt moves toward us at 60 km per second
BIt moves away at 600 km per second
CIt moves toward us at 6 km per second
DIt moves away at 60 km per second
Answer & Solution
Correct answer: D. It moves away at 60 km per second
1. The measured wavelength is longer than the laboratory one, so this is a redshift.
2. A redshift means the source is receding, not approaching.
3. The speed follows from the speed of light multiplied by the wavelength change over the rest wavelength.
4. The change is 500.1 minus 500, which is 0.1 nanometer.
5. Divide 0.1 by 500 to get 0.0002, then multiply by 300,000 kilometers per second.
6. That gives 60 kilometers per second, moving away.
7. Both toward us options ignore the direction that a longer wavelength implies.
8. The 600 kilometers per second option is out by a factor of ten in the division.
_Source: OpenStax Astronomy (CC BY 4.0), Ch 4 "Earth, Moon, and Sky" and Ch 5 "Radiation and Spectra", section 5.6 The Doppler Effect_
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