Question

Now imagine you’re an astronomer on Earth looking at the spectrum of an
object in space. In the spectrum you are looking for the so-called Lyman-alpha line of hydrogen
which occurs at a wavelength of  = 122 nm. In your telescope, you observe the Lyman-alpha
line at 243 nm. What is the speed, in units of the speed of light, of the object?

(a) The object moves away from Earth at speed v = 0.60c
(b) The object moves towards Earth at speed v = 0.60c
(c) The object moves away from Earth at speed v = 0.99c
(d) The object moves towards Earth at speed v = 0.99c