alleryStart learning →A star moving away from us shifts the wavelengths of its spectral lines in which way?
AThey get shorter
BThey vanish
CThey double
DThey get longer
Answer & Solution
Correct answer: D. They get longer
1. Motion away from an observer stretches the gaps between arriving wave crests.
2. Stretched gaps mean longer wavelengths.
3. Astronomers call that stretch a redshift, because longer means redder in visible light.
_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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