Snell's window

Snell's window

Snell's window is a phenomenon by which an underwater viewer sees everything above the surface through a cone of light of width of about 96 degrees. [cite book | title = The Underwater Photographer | author = Martin Edge and Ian Turner | year = 1999 | publisher = Focal Press | url = http://books.google.com/books?vid=ISBN0240515811&id=hg_XualqHoQC&pg=RA2-PA64&lpg=RA2-PA64&ots=EPWZUpb3gp&dq=%22Snell%27s+window%22&sig=as8s6dfwX0e8U1zT3u5lH9Xswcs | isbn = 0240515811] This phenomenon is caused by refraction of light entering water, and is governed by Snell's Law. [cite book | title = Physical Optics | author = Robert Williams Wood | publisher = The Macmillan Company | year = 1914 | url = http://books.google.com/books?id=qpjYrQxHYHIC&pg=PA66&dq=circle-of-light+water+critical-angle+180&as_brr=1&ei=G98eR9yhA5KSpgKZrrCzBg ] The area outside Snell's window will either be completely dark or will show a reflection of under-water objects.

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

Snell's window is also called 'Snell's circle' [cite book | title = Tactics for Trout | author = Dave Hughes | isbn = 0811724034 | publisher = Stackpole Books | year = 1990 | url = http://books.google.com/books?vid=ISBN0811724034&id=z-ic8uOuvUsC&dq=%22Snell%27s+circle%22] or 'optical man-hole'. [cite book | title = Color and Light in Nature | author = David K. Lynch and William Livingstone | page = 79 | year = 2001 | publisher = Cambridge University Press | isbn = 0521775043 | url = http://books.google.com/books?vid=ISBN0521775043&id=4Abp5FdhskAC&pg=RA1-PA79&lpg=RA1-PA79&ots=bM2NITwwk3&dq=%22optical+manhole%22&sig=BpZnWSAqH_WaIGOspitxGD03OZ8]

Image formation

Under ideal conditions, a fish (or person) 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 at the air/water boundary, Snell's window compresses a 180° angle of view above water to a 97° angle of view below water, which is probably the origin of the term "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). 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 in the water will veil the image behind a cloud of scattered light.

References

External links

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


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