You place the slit of your spectrograph along the log axis of a spiral galaxy an
ID: 1651612 • Letter: Y
Question
You place the slit of your spectrograph along the log axis of a spiral galaxy and take its spectrum. First you look at the spectrum of its left edge and notice a well-known dark absorption line. But instead at the wavelength of 5000 Angstroms it is at the wavelength of 5100 Angstroms. What do you conclude?
A. The left edge of this galaxy is moving towards us with the speed of 2% of the speed of light
B. The left edge of this galaxy is moving away from us with the speed of 2% of the speed of light
C. The left edge of this galaxy is at rest relative to us
D. It is not possible to make any conclusion about the speed of this galaxy or any of its parts
Explanation / Answer
wavelength,that should have been measured, theoretically, lambdao = 5000 Angstroms
measured wavelength, lambda = 5100 Angstrom
so the redshift/blue shift formulae tell us
lambda/lambdao = sqroot([1 + v/c]/[1 - v/c])
(5100/5000)^2 = (c + v)/(c - v)
1.0404c - 1.0404v = c + v
0.0404c = 2.0404v
v = 0.0198 c
so the galaxy is moving away from us at 1.98 = 2 pc the speed of light
the correct answer
B. The left edge of this galaxy is moving away from us with the speed of 2% of the speed of light
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