April 20, 2014
The photo above showing a jet contrail underlining an eye-catching display of iridescent clouds was snapped near sunset on December 13, 2013, above Turin, Italy. Diffraction of sunlight in mid-level clouds (altocumulus clouds in this case) composed of same-sized water droplets results in the pleasing pastel coloration. Because the cloud droplets are so minute, in general hundreds of times smaller than raindrops, reflection and refraction of sunlight are secondary to diffraction processes.
Photo details: Canon EOS 600D camera; 140 mm focal length; f/9 aperture; 1/320 sec. exposure time; ISO 400.