posted by Sethi .
A line in star's spectrum lies at 400.0 nanometers. In the laboratory, that same line lies at 400.2 nanometers. How fast is the star moving along the line of sight; that is, what is its radial velocity? Is it moving toward or away from us?
Since the wavelength shift is to shorter wavelengths (from 400.2 in the lab to 400.0, the star is moving toward observer (Earth).
The speed of approach along the line of sight, V, is given by
(400.2 - 400.0)/400.0 = 0.2/400 = V/c
Solve for V.