Near Sun Iridescence ~ By Tana Hojcova at Bratislava, Slovakia 4th April. In this rare image both the subtle cloud colours and the edge of the bright sun's disk have been well captured  ©Tana Hojcova, shown with permission.

About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed
Iridescent colours in clouds are produced by individual cloud droplets diffracting sunlight.

Normally we do not see anything unusual because the droplets are of different sizes and scatter, diffract, light differently resulting in near white where the diffraction patterns overlap. Additionally, in thick clouds rays are scattered by many droplets before they emerge from the cloud.

When, as here, the cloud is thin and the droplets reasonably monosized locally then we see the diffracted light colours.

The most intense diffraction is through small angles and therefore it appears most often close to the sun. This is the best place to search but give priority to care of your eyes. Hide the sun behind the edge of a building or a tree. Alternatively look at the reflection of the sky in a dark mirror. A convex sunglass lens used as a mirror works well. A pool of wate is also a good dark mirror.

In some directions the crests of scattered waves of a particular colour overlap to give bright light. In other directions the wave crests are out of phase, they cancel each other and there is darkness.