Correct Response: A. When sunlight strikes Earth's atmosphere, it is scattered by molecules in the atmosphere. These molecules are much smaller than the wavelengths of visible light in sunlight. This is called Rayleigh Scattering and it selectively affects shorter wavelengths at the blue-violet end of the visible spectrum more than longer wavelengths. As a result, blue wavelengths are much more widely scattered across the sky than other wavelengths, giving the sky its blue color. Although some long wavelengths in sunlight are reflected, reflection of longer wavelengths (B) is not responsible for the sky's color. Sunlight is refracted as it passes through water droplets, but it is not refracted by gaseous water vapor (C) and therefore does not affect the sky's color. While water vapor accounts for up to 70% of the absorption of incoming sunlight, its selective absorption of different wavelengths does not cause the sky to appear blue (D).
