Velocity addition: A star is moving towards the earth with a speed at 90% the speed of light. It emits light, which moves away from the star at the speed of light. Relative to us on earth, what is the speed of the light moving toward us from the star?
A) 0.90c
B) c
C) 1.1c
D) 1.20c
E) 1.9c

Question

Quizplus · Accepted Answer

According to the principle of velocity addition, the velocity of the light relative to us on earth will be the sum of the speed of light and the speed of the star relative to us on earth. Therefore, the velocity of light moving towards us will be c + 0.9c = 1.9c. However, according to the theory of relativity, nothing can move faster than the speed of light. Therefore, the actual velocity of light moving towards us from the star will be c.

Velocity Addition: a Star Is Moving Towards the Earth with a Speed