A quasar (quasi-stellar object) travels away from the Earth at half the speed of light. What is the speed, with respect to the Earth, of the light we detect coming from it?

In accordance to Einstein’s first postulate of special theory of relativity, the fundamental laws of physics have the same form for all inertial reference frames. But according to second postulate, the speed of light in free space has the same value c in all inertial reference frames that signifies that the velocity of light in vacuum is independent of the relative motion of the source and the observer. A quasar travels away from the Earth at half the speed of light. Therefore the speed of light we detect coming from the quasar with respect to the Earth will be c.