SNELL’S LAW and WINDOW
The Objects in the water appear closer and bigger
Snell’s Window is one of the coolest underwater phenomena you’ll find. It is a trick of physics that allows you to see everything above the surface through a cone of light with a width of about 96 degrees. The reason this happens is called refraction. When light enters water, it bends due to something called Snell’s Law. This law states that light (or other waves), will change their angle of incidence when passing through the boundary of two different mediums. In this case we are talking about passing from air into water.
The physics behind Snell’s Window, which was named after the Dutch mathematician Willebrod Van Roigen Snell (1580–1662), a professor of mathematics at Leiden University in the Netherlands—but you knew that
On your next dive trip, wait for a day with calm conditions to try this experiment. While hanging on the anchor line at the end of a dive look up during your safety spot. At a depth of 10 feet, you will probably be a little too deep for a perfect window, but slowly ascend and keep looking up. Between three and six feet, the entire surface, from horizon to horizon, will be clearly visible! Aside from the amazing physics at work, think of all the incredible photographic opportunities this presents! Here, then, are some techniques to keep in mind while shooting using Snell’s Window.
Vision
Vision is affected greatly underwater. Objects will appear about a third larger and closer underwater. This is because of the way light refracts, or bends, through the mask lens. The magnification of an object depends on the following.
- Turbidity of the water
- Design of the mask lens
- Distance from the mask lens to the diver’s eyes
You need a mask because your eyes are designed to resolve an image in air. The ability of human eyes to focus depends on how the light enters the eye. When underwater, light enters a diver’s eyes at a different angle, and if the scuba mask did not have an air gap, the diver would see objects that were out of focus. New divers may have a tough time adjusting to the magnification of near objects when underwater. After a few dives, they may hardly notice its effect.
Look at the image of the pencil in the glass of water. You can see how the pencil appears to be larger in the water.
Color
Color becomes visible when certain wavelengths of light reflect off an object. Colors change quite dramatically underwater. The wavelengths of light bend or are filtered out. Long wavelengths, such as red and orange, are absorbed by water. Short wavelengths, such as blue, are scattered. This gives clear water its blue appearance.
In murky water, there is less light penetration. Things have a greenish-yellow appearance. If you were to cut yourself, the blood will look green or brown. Even under the best conditions, colors will fade the deeper you dive. The way colors are absorbed impacts underwater photography. A good light source is needed for vibrant images. Photographers often use a red filter to get images with deep natural light. The filter helps remove some of the blue color.
The sun’s angle also has a direct effect on visibility. The water surface can behave like a mirror. When the sun is directly overhead, most of the light will enter the water. But at dawn and dusk, the light may reflect off the surface. It may also not penetrate the water.
