One of the absorption lines of hydrogen has a rest wavelength of 656 nm (at rest means with respect to the observer, like in a lab). You observe several stars and measure the wavelength of this same hydrogen absorption line in each star. Based on the measured wavelength of this line, which star is moving towards the Earth the fastest?
A)630 nm
B)670 nm
C)640 nm
D)680 nm
E)656 nm

Question

Quizplus · Accepted Answer

The star with the absorption line at 630 nm is moving towards the Earth the fastest because its wavelength is the most blueshifted (shortest wavelength) from the rest wavelength of 656 nm, indicating motion towards the observer.

One of the Absorption Lines of Hydrogen Has a Rest