Red Shift/ Blue Shift

[fingerscrossedd]

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From AAMC 4, number 15, which has been asked before but none of the explanations really clarified it for me:

15. An astronomer observes a hydrogen line in the spectrum of a star. The wavelength of hydrogen in the laboratory is 6.563 x 10^-7 m, but the wavelength in the star's light is measured at 6.56186 x 10^-7 m. Which of the following explains this discrepancy?

A) The star is moving away from earth
B) The wavelength of light that the star is emitting changes frequently
C) The frequency of light that the star is emitting changes frequently
D) The star is approaching earth



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My thinking:
-wavelength is longer for the observer
-source is moving away (A)

Thanks for the help! 😍😍😍

 

[milski]

The wavelength is shorter for the observer.

6.563x10^-7 is the observed wavelength from hydrogen in a laboratory - that would be the same wavelength as it is for the transmitter.

The observed wavelength is 6.56186x10^-7, which is shorter than the wavelength form a stationary source (in the lab), thus the source is moving towards the observer.

 

[fingerscrossedd]

Oh, wow, I had a tough time realizing that the "lab" wasn't the "observer" - it represents the wavelength value from the source, yes? Thanks a lot!