June 18, 2014
Looking up from underwater, one sees the whole sky. But it doesn’t stretch 180 degrees from horizon to horizon like it does above water. Instead it's compressed into a circle about 97 degrees across, regardless of the observer’s depth. This occurs because light rays bend when entering or exiting water. The shrunken sky seen by submerged observers is called Snell’s Window, informally named for Willebrord Snellius, a Dutch astronomer and mathematician. It's also called the optical manhole.
In this wide-angle picture taken at French Cay, Turks and Caicos, Snell’s Window shows tropical clouds, the Sun at the bottom and a few bubbles from the scuba diver. The window also shows ragged edges, with bits of sky detached from the main window. These are due to surface waves. Seen through perfectly flat water, the edges are sharp. Photo taken on April 22, 2014.
Photo details: Nikon D600 camera; Tokina fisheye lens at 14mm; in Hugyfot housing; 1/100 exposure; f/25; ISO 320.