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In this slide, a diver is underwater and looking at the sky. At angles smaller than 97.2 degrees, there is no transmission, only reflection. This is called total internal reflection, and explains why there are dark regions in the picture outside of the cone.


Optical manhole is a phenomenon by which an underwater viewer sees everything above the surface through a cone of light of width of about 96 degrees. This phenomenon is caused by refraction of light entering water, and is governed by Snell's Law. The area outside Snell's window will either be completely dark or show a reflection of underwater objects by total internal reflection.


More details on optical manhole/snell's window:'s_window


Here are a few really cool sites about how fish see as it pertains to Snell's Window, and the "science of stalking fish"! ~