A spaceship, moving away from the Earth at a speed of 0.9c, shines a green laser backwards at Earth. An observer on Earth would see the light arriving at a speed of
A) 0.1c and with an inverted frequency.
B) more than c but less than 1.9c.
C) c but at a shorter wavelength.
D) c but at a smaller frequency.
E) 1.9c and with a shorter wavelength.

Question

Quizplus · Accepted Answer

According to the principle of relativity, the speed of light is constant in all inertial frames of reference. Therefore, an observer on Earth would also measure the speed of the green laser to be c. However, due to the Doppler effect, the frequency of the light observed on Earth would be smaller than the emitted frequency, resulting in a shift towards the red end of the spectrum. Therefore, the correct answer is option D.

A Spaceship, Moving Away from the Earth at a Speed