If a hydrogen emission line appears at 656.2 nm in the laboratory, what can we say about a cloud of hydrogen gas with this same emission line at 680.2 nm?
A)It is neither receding nor approaching us.
B)The location of the emission line has nothing to do with speed.
C)It is approaching us.
D)It is receding away from us.

Question

Quizplus · Accepted Answer

The wavelength of light emitted by a source is shifted if the source is either approaching or receding away from us relative to the laboratory where the wavelength was measured. This effect is known as the Doppler shift. The shift depends on the relative speed of the source and observer, as well as the wavelength of the emitted light. A redshift (increase in wavelength) occurs if the source is receding away from us, whereas a blueshift (decrease in wavelength) occurs if the source is approaching us. In this case, because the laboratory measurement is at a shorter wavelength (656.2 nm) than the wavelength emitted by the cloud (680.2 nm), the cloud must be receding away from us. Therefore, the answer is D.

If a Hydrogen Emission Line Appears at 656