Sunlight contains every visible color, each with its own wavelength. Light travels as ripples, with crests and valleys; wavelength is the distance between the crests. The shorter the wavelength, the steeper the ripples. Of the three primary visual colors—blue, red, and green—blue has the shortest wavelength, and red the longest. As sunlight hits Earth, blue light’s steep waves cause particles in the air to scatter it almost totally, turning our skies cerulean. The leftovers combine to create the sun’s yellow glow.