The sky appears blue to human observers because blue light from the sun is scattered in all directions by the gases and particles in the Earth's atmosphere. This scattering causes the direct sunlight to appear white, but the sky itself takes on a blue hue because the blue light is scattered more efficiently than the other colors in the spectrum. This effect is known as Rayleigh scattering, named after the English physicist Lord Rayleigh who first described it in the 19th century.