The red shift A light source recedes from an observer with a speed vsource that is small compared with c.
(a) Show that the fractional shift in the measured wavelength is given by the approximate expression ∆A/A ≈ vsource/c this phenomenon is known as the red shift, because the visible light is shifted toward the red.
(b) Spectroscopic measurements of light at A = 397 nm coming from a galaxy in Ursa Major reveal a red shift of 20.0 nm. What is the recessional speed of the galaxy?