1. The problem statement, all variables and given/known data A spaceship moves radially away from the earth with an acceleration of 20m/c2. How long does it take the sodium street lamps ([tex]\lambda[/tex]=589 nm) on earth to be invisible (with a powerful telescope) to the human eye of the astronauts? The visible spectrum is 400-700nm. 2. Relevant equations fobs = ((1+(v/c))1/2/(1-(v/c))1/2) fsource 3. The attempt at a solution I can use the doppler shift equation, to solve for velocity, but what should I use for fobs? Should I use 400nm or 700nm. I'm thinking 400 nm because of length contraction... After I find a velocity, I can use the acceleration given and solve for time.