A distant star emits light with a peak intensity at 620 nm. If the star is moving toward Earth at 0.024c, what is the wavelength of the peak intensity as measured by an Earth observer?
A) 605 nm
B) 591 nm
C) 650 nm
D) 635 nm

Question

Quizplus · Accepted Answer

The wavelength of light emitted by a moving source is affected by the Doppler effect, given by:
λ_obs = λ_em/(1 + v/c)  (where v is the velocity of the source, c is the speed of light, λ_obs is the observed wavelength and λ_em is the emitted wavelength)
Here, the emitted wavelength is given as 620 nm, the velocity of the source towards Earth is 0.024c. 
Plugging in the values we get, 
λ_obs = 620/(1+0.024) = 605 nm
Hence, the wavelength of the peak intensity as measured by an Earth observer is 605 nm.

A Distant Star Emits Light with a Peak Intensity at 620