Two spaceships are traveling past Earth at 90% of the speed of light in opposite directions (i.e., they are approaching each other). One spaceship turns on a searchlight, which is seen by scientists aboard the other. What speed do the scientists measure for this light (c = speed of light in a vacuum)?
A) 1.9 c (equal to c + 0.9 c)
B) 1.8 c (equal to 2 *0.9 c).
C) c
D) 0.9 c

Question

Quizplus · Accepted Answer

According to Einstein's theory of relativity, the speed of light in a vacuum is constant and does not depend on the speed of the observer or the source of the light. Therefore, regardless of the relative motion of the source and the observer, the scientists will measure the speed of light as c.

Two Spaceships Are Traveling Past Earth at 90% of the Speed