On 1666, sir Isaac Newton, put a prism in front of sun's rays and discovered that white light is divided into seven colors or the visible spectrum : The energy of light travels in waves . Some light travels in short "choppy" waves. Other light travels in long "lazy" waves. Blue light's waves are shorter then Red light waves. All light travels in a straight line unless something gets in the way. Sunlight reaches Earth's atmosphere and it scatters (deflects) in all directions by the gases, particles and tiny molecules in air. The Blue light is scattered more than other colors because it travels as shorter and smaller waves. This why we see the sky blue most of the time !!