Excited hydrogen emits a spectrum of wavelengths in which the hydrogen-alpha line
PHYSICS · Single correct · A Level · Physics · Astrophysics · Astrophysics
Problem
Excited hydrogen emits a spectrum of wavelengths in which the hydrogen-alpha line, at a wavelength of 656.28 nm, is a dominant feature. When a distant galaxy is observed from the Earth, the hydrogen-alpha line has a wavelength of 670.0 nm. If c is the speed of light, which of the following best describes the motion of the distant galaxy relative to the Earth?
(1) The galaxy is moving towards the Earth with a speed of 2% of c .
(2) The galaxy is moving towards the Earth with a speed of 23.7% of c .
(3) The galaxy is moving away from the Earth with a speed of 2% of c .
(4) The galaxy is moving away from the Earth with a speed of 23.7% of c .