A phenomenon that causes the sky to be blue and the sun to be yellow.

Gaseous molecules in the atmosphere contain positively charged elements in their nuclei and negatively charged electrons in their electron cloud. The electric bond that exists between these elements can be crudely modelled as an electric dipole.

If an electric dipole is hit by electromagnetic radiation ( such as light ) of a frequency that is close to the natural frequency of its electric bond it may absorb the radiation and move to a higher energy state.

After a short time it may then decay back to its original more stable energy state and re-emit the light it absorbed. The light will be re-emitted in a random direction except along the axis of oscillation. ( This btw is why sunlight is polarised in some sections of the sky - you are looking along one of the axes of oscillation for the scattered light)

Thus the overall effect of this is that light of the right frequency is scattered away from its original direction of travel. It just so happens that for the bonds present in the molecules that make up our atmosphere the right frequency is that of blue light.