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Snell's window
Underwater phenomenon due to Snell's Law / From Wikipedia, the free encyclopedia
Snell's window (also called Snell's circle[1] or optical man-hole[2]) is a phenomenon by which an underwater viewer sees everything above the surface through a cone of light of width of about 96 degrees.[3] This phenomenon is caused by refraction of light entering water, and is governed by Snell's Law.[4] The area outside Snell's window will either be completely dark or show a reflection of underwater objects by total internal reflection.
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Underwater photographers sometimes compose photographs from below such that their subjects fall inside Snell's window, which backlights and focuses attention on the subjects.