"Crepuscular rays" or "twilight rays" are alternating light and dark zones in the atmosphere radiating from the sun. They arise when the direct solar radiation is blocked by clouds, for example. The diverging rays become visible, because they are scattered at air molecules and aerosol or cloud particles. So the rays which are initially directed into the other direction are re-directed to the eye of the observer. Thereby the contrast between light and dark zones is enhanced by possibly occurring haze or droplets.
Iridescence often occurs at the edge of the blocking clouds (see chapter "Iridescence"). It can be also noticed in Rays9.jpg, Rays13.jpg and Rays14.jpg. At the edge of the clouds appears also a light zone resulting from iridescence. However the colours are not separated and therefore not visible, since cloud droplets are too large and their sizes vary too much in these cloud regions.

The blue shadows in the centre and at the left edge of CrepuscRays4.jpg are generated by invisible cloud parts, which throw their shadow into direction of the observer. In CrepuscRays4.jpg the diverging rays are below the cloud. In former times one wrongfully believed that "the sun pulled water", at least above the ocean.
Crepuscular rays often occur at sunrise or sunset when a mountain chain shades the direct sunlight, for example.

Rays1-Rays14.jpg: S. Borrmann, Lighthouse near Fajardo, Puerto Rico during the RICO-PRACS Field Campaign, 11 December 2004, 4:44 p.m. to 5:03 p.m.