Why is the color of the sky blue? The Earth has an atmosphere made of gaseous particles. When light from the Sun strikes these gas molecules, it gets scattered. By the quantum laws, light can behave both as a particle and wave. This nature depends on the typical dimensions of the obstacle in its path. If the dimensions of this object are much smaller than the wavelength of light, it behaves as a wave. If the dimensions are comparable, it behaves like it is composed of particles called photons. Gas molecules in the atmosphere are much smaller than the typical wavelength of visible light. Therefore, the behavior of light is wave like. In these circumstances, the scattering in effect is Rayleigh scattering. It says that intensity of transmitted light power is inversely proportional to the fourth power of the wavelength of the light. Therefore, shorter wavelengths are scattered much more strongly than longer wavelengths. In the spectral decomposition of visible light (VIBGYOR), blue possess the least wavelength hence it scattered the most. This is why the sky appears blue. This fact is also easily demonstrated by the fact that the Sun itself appears red around sunset and sunrise. This is understood by the same logic as above. Most of the blue and shorter wavelengths are scattered away, leaving just red, when the Sun is at the horizon. At the horizon, light from the Sun has to travel the longest distance through the atmosphere. This gives more leeway to Rayleigh scattering, making the Sun appear Red.