Answer: Scattering of light
Notes: The sky appears blue because the molecules in the air scatter light in the higher wavelengths (that is, the blue light), while lower wavelength light (that is, the light on the red end of the spectrum) goes through to the ground. So the light that get finally reflected down from the scattering is blue, therefore a blue sky. This is called Rayleigh scattering. Rayleigh scattering, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light.
