Suppose a rocketship is receding from the earth at a speed of 2/10th the velocity of light. A light in the rocketship appears blue to the passengers on the ship. What colour would it appear to an observer on the earth?

The correct answer is C because of the Doppler Effect for light. When a light source moves away from an observer, the wavelength of the light increases and its frequency decreases. This shift toward longer wavelengths is known as a redshift. In the visible light spectrum, blue has a shorter wavelength, while colors like yellow, orange, and red have increasingly longer wavelengths. Since the rocketship is receding from the Earth at a significant speed, the blue light emitted from the ship will be stretched toward the red end of the spectrum. At a speed of 2/10th the velocity of light, the shift is sufficient to make the light appear yellow to an observer on Earth. Orange and red would represent even greater shifts or higher speeds.