OPOD - Iridescence and diffraction, California

Iridescence writes large in the sky that light has wave properties and that there are small things, be they tiny water droplets, ice crystals or even pollen grains, to scatter it.

Imagine (far left) plane waves of blue sunlight scattering from a cloud water droplet. Outgoing spherical waves radiate out from every point on the drop�s rim. For simplicity the diagram shows waves from only two points. In some directions the outgoing waves are in-phase. and radiate blue light � in others they are out of phase, cancel, and there is no light. The process is interference or diffraction � the two terms are more or less synonymous.

At right is the exact same geometry but with longer wavelength red light. There are different directions for brightness and darkness. A cloud of similar size droplets will appear red in some directions and different colours elsewhere.

Different viewing angles, sun direction and local variations in cloud droplet size all influence the colours.