When light is emitted from the sun, it is emitted at all visible wavelengths of light, and some non-visible wavelengths. When the visible wavelengths overlap each other they create white light. When light passes through a relatively clear sky, light particles or photons necessarily come into contact with molecules of oxygen, nitrogen, water vapor and other substances in the air. Light of shorter wavelengths (blue, violet) tend to reflect more from impacting with these molecules than light of longer wavelengths (red, yellow, orange) which tend to pass through easier. A small amount of blue light gets scattered, thus making the sky blue.