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Introduction to the eye and how it is effected in an underwater environment.
Parts of the Eye
- Cornea -- the clear front window of the eye. The cornea transmits and focuses light into the eye.
- Iris -- the coloured part of the eye. The iris helps regulate the amount of light that enters the eye.
- Pupil -- the dark centre in the middle of the iris. The pupil determines how much light is let into the eye. It changes sizes to accommodate for the amount of light that is available.
- Lens -- the transparent structure inside the eye that focuses light rays onto the retina.
- Retina -- the nerve layer that lines the back of the eye. The retina senses light and creates impulses that are sent through the optic nerve to the brain.
- Macula -- a small area in the retina that contains special light-sensitive cells. The macula allows us to see fine details clearly.
- Optic Nerve -- the nerve that connects the eye to the brain. The optic nerve carries the impulses formed by the retina to the brain, which interprets them as images.
- Vitreous -- the clear, jelly-like substance that fills the middle of the eye.
Vision
 Objects underwater look about 25% bigger and closer than they really are. If you are under water and your eyes are in direct contact with the water, objects would not be magnified, however to see, divers eyes have to be separated from the water by a mask or goggles.
If you look into the water from above, objects are not where they appear to be. This is because light rays are refracted (bent) when passing from one medium into another. Put a pencil into a glass of water so that some sticks out and then look at it through the side of the glass. You'll note that the pencil appears to have a kink in it where it intersects the surface of the water.
To see something, light must travel from what you see into your eyes. When you see something under water, light travels differently from seeing it in the air. When the light passes from the water to the air, it acts like a magnifying glass. Everything underwater looks a little closer than if the water was not there. Looking closer makes the underwater things look bigger.
The"virtual image" that your brain constructs from the composite view of both eyes is about 7% smaller and 33% closer than the real image in the water. Hence, the old adage that things look about 25% bigger underwater.
Colours
Many of us can remember the acronym "Richard of York Gains Battle in Vain, which identifies the colours of the rainbow; Red, Orange, Yellow, Green, Blue, Indigo, and Violet. Collectively these colours make white light.
As most divers will know the deeper you go the more the terrain looks blue/green. This is because the water acts as a filter. White light from the sun hits the surface of the water, but as it goes deeper the colours are filtered out (in the order above), one by one.
Just under the surface colours are ok, but at 10m the Red, some orange and yellow will have gone. At about 30m most of the yellow and orange are gone, leaving the greens and blues. You can see that the colours are still there by shining your torch at depth.
Deeper still and the blues and violets will disappear and eventually all light will be filtered out.
This loss of colour can lead to some fascinating effects; photographs taken without a flash will appear greeny blue, switch the flash on and hey presto! beautiful colours. The light from a camera flash or torch is also subject to this colour filtration, so it too will lose its colours over distance.
If a diver cuts himself at 20m the blood will be greeny black as there is no red light.
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