Refraction and Diffraction
December 01, 2009
The above photo shows both a classic 22 degree halo and cloud iridescence (irisation) – in the cloud to the right of the Sun. Whereas, halos form from refraction in hexagonal ice crystals found in cirrus type clouds, iridescence is caused by diffraction of light in water droplets, which typically compose mid-level clouds. Observe the pink and blue pastel hues in the iridescent cloud. Since light waves overlap more when they’re deflected than when they’re refracted, the colors are more smeared and therefore less pure. Also of note is the barn swallow to the left of the Sun and a contrail nearly tangent to the halo. Photo taken from Almada, Portugal on April 19, 2009 at 4:07 p.m.
Photo details: Canon 400D camera; ISO 400; F-11; Exposure 1/1000 second; 18 mm lens.Almada coordinates: N38.666667, W9.15