Iridescence over Yunnan Observatory
Jeff Dai captured these beautiful iridescent clouds at Yunnan Astronomical Observatory, China. Yunnan Observatory has a 2.4m optical and 40m radio telescopes and is part of the National Astronomical Observatories of the Chinese Academy of Sciences.

All images ©Jeff Dai, shown with permission
Individual water droplets in the clouds scatter sunlight, mainly from their outer skin.

Each point on the skin is effectively a source of outgoing spherical waves. The waves overlap and interfere to give brightness in some directions and darkness in others. The interference is wavelength dependent and the scattering or diffraction pattern is coloured.

A cloud of uniform sized droplets would give a corona. When the droplet size varies across the cloud we see more disordered colours - iridescence.
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