Question

The wavelength of light emitted from a distant galaxy by a hydrogen atom is observed to be 400 nm. If it is known that the laboratory wavelength of this light emitted by hydrogen is 380 nm, what is the velocity of the galaxy?

(a) .135c away from Earth
(b) .135c toward Earth
(c) .055 away from Earth
(d) .055 toward Earth

Answer 1

Redshifts, as is the case here, are away from the observer, and the wavelength shift is approximately
deltaL/L = v/c = 20/380 = .053

I used an approximate formula that is easier for me to remember than the correct relativistic Doppler formula, which you can find here:

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/reldop2.html

Go with (c)