Snell's window
Encyclopedia
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. 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 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' or 'optical man-hole'.
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
. 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.
Underwater vision
Light rays bend when they travel from one medium to another; the amount of bending is determined by the refractive indices of the two media. If one medium has a particular curved shape, it functions as a lens. The cornea, humours, and crystalline lens of the eye together form a lens that focuses...
sees everything above the surface through a cone of light of width of about 96 degrees. This phenomenon is caused by refraction
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
of light entering water, and is governed by Snell's Law
Snell's law
In optics and physics, Snell's law is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water and glass...
. The area outside Snell's window will either be completely dark or will show a reflection of under-water objects
Total internal reflection
Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is...
.
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' or 'optical man-hole'.
Image formation
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 refractionRefraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
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
Fisheye lens
In photography, a fisheye lens is a wide-angle lens that takes in a broad, panoramic and hemispherical image. Originally developed for use in meteorology to study cloud formation and called "whole-sky lenses", fisheye lenses quickly became popular in general photography for their unique, distorted...
. 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
Fresnel equations
The Fresnel equations , deduced by Augustin-Jean Fresnel , describe the behaviour of light when moving between media of differing refractive indices...
). 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
Turbidity
Turbidity is the cloudiness or haziness of a fluid caused by individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality....
in the water will veil the image behind a cloud of scattered light.