Dispersion is used to study the interaction between light and
particles of big size. We call a "big particle" when
its size is bigger than a wave length. A big particle behaves like
a mirror in the atmosphere, without any preference of any colour
contained in the incident white light.
type of interaction is the one that takes place between the sunlight
and the clouds in the sky, since the clouds are composed by big
sized non-coloured drops of water. These drops reflect as a mirror
the white polychrome light that interacts with them, without changing
the light colour. This is the reason because clouds appear white
coloured in the sky.
some particles of the air have a similar size of one wave length,
of any colour, then they spread the light in a different way, irradiating
a big range of different colours. This is the origin of the rest
of the colours that we can see in the sky.