Why is the sky blue?

In the sunny day, you look the blue sky above. Have you ever asked yourself why the sky is blue?

The answer to that question is the light and the atmosphere. Light is a kind of energy that radiates, or travels, in waves. Light from the sun or from your study lamps looks white but it is actually the combination of many colours, seven basic colours: red, orange, yellow, green, blue, indigo and violet. Each type of light has its own characteristic included wavelength, frequency and energy it contains. The energy of radiation depends on its wavelength and it frequency. The longer the light wavelength is, the lower the frequency and the less energy it contains.

Of all the lights above, the red one has the longest wavelength but lowest frequency and energy. The atmosphere contains the mixture of gas molecules and other materials surrounding the earth. When the light travels to the earth, the light hits some dust and gas molecules. The gas molecules in the atmosphere scatter the higher-energy (high frequency) blue portion of the sunlight more than they scatter the lower-energy red portion of the sunlight (this is called Rayleigh scattering, named for the physicist Lord John Rayleigh). The blue light scatter against in all direction, you see the blue everywhere overhead and as a result the sky looks blue.