An intergalactic cruiser is approaching a hostile spacecraft. Both vehicles move at a constant velocity. The velocity of the cruiser relative to the spacecraft is vCS=+0.7c, the direction to the right being the positive direction. The cruiser fires a beam of laser light at the hostile renegades. The velocity of the laser beam relative to the cruiser is vLC=+c.
a) What is the velocity vLS of the laser beam relative to the renegades’ spacecraft?
b) What is the velocity v at which the renegades see the laser beam move away from the cruiser?
The Speed-of-Light Postulate: The speed of light in a vacuum, measured in any inertial reference frame, always has the same value of c, no matter how fast the source of light and the observer are moving relative to each other.
The Attempt at a Solution
In both cases, it is asked how the speed of light is percieved. Question a) makes sense, as it's asking for the speed of light relative to an inertial reference frame. According to The Speed-of-Light Postulate of special relativity, light travelling in vacuum percieved in an inertial reference frame always has the value of c so the answer is vLS=+c. What I'm wondering about is question b). In order to answer this question I need to understand the following:
How is question b) different from question a)? In question b) it is also being asked how the speed of light is percieved viewed from an inertial reference frame, so according to the abovementioned postulate it should also be +c. But that is not the correct answer. Why is it not that?