1. The problem statement, all variables and given/known data An astronomer observes a hydrogen line in the spectrum of a star. The wavelength of hydrogen in the laboratory is 6.563 x 10-7m, but the wavelength in the star’s light is measured at 6.56186 x 10-7m. Which of the following explains this discrepancy? A) The star is moving away from Earth. B) The wavelength of light that the star is emitting changes constantly. C) The frequency of light that the star is emitting changes constantly. D) The star is approaching Earth. I chose A, the star is moving away from the observer. D is correct. My reasoning: Since the photon's wavelength observed in the laboratory (lambda') is greater than the actual wavelength (lambda) issued from the star, I thought this meant the star was moving AWAY. The wavelength is "stretched out" as the star recedes from the observer, and the observed frequency is smaller. Am I switching around the the Doppler effect on wavelength and frequency? Could someone clarify the whole concept here, which I must have misunderstood. (The effect above is called a blue shift-- but I'm not sure why.) 2) This question makes me wonder if, for photons with wavelength in the visible range, can the Doppler effect have an effect on the color perceived?? In other words, if an object emitting yellow light were receding from the observer, might the light actually be perceived as green or blue (ie shorter wavelength)? Thanks a lot.