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Homework Help: Can a beam of light appear to have different trajectories?

  1. Oct 5, 2016 #1
    1. The problem statement, all variables and given/known data
    In an inertial reference frame, a beam of light is shone 30 degrees from the x-axis. What is the speed of another inertial reference frame along the x-axis where the beam of light is 90 degrees from the x-axis?

    2. Relevant equations

    Can't really think of any equations that would be applicable to this.

    3. The attempt at a solution

    My reasoning is that this is impossible, which is an option. The speed of light is c in all inertial reference frames, and to the observer in the primed frame, the light would not have its trajectory changed as it is always going at c with respect to them. One of the options is c, which I can understand being the answer as the light would not be moving with respect to the observer in the x-axis, and would have to be in the y-axis. But isn't this impossible - no reference frame can move at c?
  2. jcsd
  3. Oct 6, 2016 #2


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    2017 Award

    Staff: Mentor

    The speed of light cannot change, but its direction (its velocity) does if you change the reference frame.

    This might be easier to see with the opposite direction: Have a beam of light shine orthogonal to the x-axis in reference frame A, now move along the x-axis. Do you expect the light to be still orthogonal to the x-axis? Where would it hit a distant wall in that case, and does that agree with frame A?
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