According to the principles of special relativity, the speed of light in a vacuum, denoted by 'c,' is constant and the same for all observers, regardless of their relative motion. This means that no matter how fast a spacecraft is moving relative to the source of light, the measured speed of light will always be 'c' for all observers.
In your scenario, if a spacecraft is traveling at 50 percent of the speed of light relative to the source of light, both an observer on the spacecraft and an observer at rest relative to the source of light would measure the speed of light to be 'c.' This is a fundamental postulate of special relativity and has been experimentally verified numerous times.
It's important to note that the constancy of the speed of light is a foundational aspect of the theory of special relativity and has far-reaching implications, such as time dilation and length contraction, which arise due to the relative nature of space and time as described by the theory.