# Snell's window

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{{short description|Underwater phenomenon due to Snell's Law}}
thumb|A diver viewed from below who appears inside of Snell's window.
'''Snell's window''' (also called '''Snell's circle'''<ref>
{{cite book
 | title = Tactics for Trout
 | author = Dave Hughes
 | isbn = 0-8117-2403-4
 | publisher = Stackpole Books
 | year = 1990
 | url = https://archive.org/details/tacticsfortrout00hugh
| url-access = registration
 | quote = Snell's circle.
 }}</ref> or '''optical man-hole'''<ref>
{{cite book
 | title = Color and Light in Nature
 | author = David K. Lynch and William Livingstone
 | page = 79
 | year = 2001
 | publisher = Cambridge University Press
 | isbn = 0-521-77504-3
 | url = https://books.google.com/books?id=4Abp5FdhskAC&q=%22optical+manhole%22&pg=RA1-PA79
}}</ref>) is a phenomenon by which an [underwater viewer](/source/underwater_vision) sees everything above the surface through a cone of light of width of about 96 degrees.<ref>{{cite book | title = The Underwater Photographer | author = Martin Edge and Ian Turner | year = 1999 | publisher = Focal Press | url = https://books.google.com/books?id=hg_XualqHoQC&q=%22Snell%27s+window%22&pg=RA2-PA64 | isbn = 0-240-51581-1}}</ref> This phenomenon is caused by [refraction](/source/refraction) of light entering water, and is governed by [Snell's Law](/source/Snell's_Law).<ref>{{cite book | title = Physical Optics | author = Robert Williams Wood | publisher = The Macmillan Company | year = 1914 | url = https://archive.org/details/physicaloptics01woodgoog | page = [https://archive.org/details/physicaloptics01woodgoog/page/n88 66] | quote = circle-of-light water critical-angle 180. }}</ref> The area outside Snell's window will either be completely dark or show a reflection of underwater objects by [total internal reflection](/source/total_internal_reflection).

Underwater photographers sometimes compose photographs from below such that their subjects fall inside Snell's window, which backlights and focuses attention on the subjects.

== Image formation ==
{{multiple image
 | align = left
 | direction = vertical
 | width = 400
 | footer =
 | image1 = RefractionReflextion.svg
 | alt1 = Image one: light coming up from the water at a steep angle passes through, bent outwards away from the vertical. Image two:light hitting the surface at the critical angle is bent to pass along the water's surface. Image three

 | alt2 = 
 | caption2 = View straight up from underwater. The above-water hemisphere is visible, compressed (as by a circular [fisheye lens](/source/fisheye_lens)) into a circle (Snell's window) bounded by the critical angle. Everything outside the critical-angle circle is reflected from below the water.
 }}
thumb|right|The edge of Snell's window, in this case the boundary between reflected bottom (teal) and refracted sky and above-water structures (blue and gray)Under ideal conditions, an observer looking up at the water surface from underneath sees a perfectly circular image of the entire above-water hemisphere—from horizon to horizon. Due to [refraction](/source/refraction) at the air/water boundary, Snell's window compresses a 180° angle of view above water to a 97° angle of view below water, similar to the effect of a [fisheye lens](/source/fisheye_lens). The brightness of this image falls off to nothing at the circumference/horizon because more of the incident light at low grazing angles is reflected rather than refracted (see [Fresnel equations](/source/Fresnel_equations)). Refraction is very sensitive to any irregularities in the flatness of the surface (such as ripples or waves), which will cause local distortions or complete disintegration of the image. [Turbidity](/source/Turbidity) in the water will veil the image behind a cloud of scattered light.

==References==
{{reflist}}

== External links ==
* [http://www.daveread.com/uw-photo/comp101/snells_window.html Explanation of the physics behind Snell's window]
* [https://www.flickr.com/photos/gerb/196296131/ Under-water photograph showing Snell's window]

{{optics-stub}}
Category:Water
Category:Geometrical optics

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Adapted from the Wikipedia article [Snell's window](https://en.wikipedia.org/wiki/Snell's_window) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Snell's_window?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.