Iridescence and Corona above Curitiba, Brazil
June 20, 2007
Water droplets in altocumulus clouds can interact with white sunlight, revealing its wave nature. Each droplet behaves identically, as if it was a circular slit in an opaque obstacle. When droplets aren't of a uniform size, all diffracted wavelengths overlap, and clouds will appear in white or light gray to an observer on the Earth's surface. However, if similar-sized droplets fill the sky around the Sun, metallic hues can be observed, like those exhibited by the iridescent clouds and corona pictured above. Caution is strongly recommended in observing or photographing the brighter portion of the sky near the solar disk. On the above picture, taken in Curitiba, Brazil (on December 3, 2006), the Sun was shielded by a corner of residential building named “Our Lady of Aparecida,” the official Patron Saint of Brazil.