The amount light is focused when it passes through the cornea is due to the difference in density between air and the cornea. The cornea has a density almost the same as water and therefore the difference in density between water and the cornea is very small. When light passes from water to the cornea, very little focusing occurs and the light will not be in focus until it is behind the retina. Therefore, when we open our eyes underwater everything appears blurry!

The solution is to wear a dive mask. The dive mask creates airspace in front of the cornea, allowing the normal amount of focusing to occur when light passes from the air to the cornea, giving us the same vision as on the surface. Nov that we can see clearly, we notice that everything appears larger underwater than it does on land. What is going on?

The glass in the dive mask has almost the same optical properties as water and therefore very little focusing occurs as light passes front the water into the mask lens. However, the density of the mask lens is much greater than the density of the air in the mask. As light passes from the mask lens into the airspace in the mask, it diverges (the opposite of focusing)! This results in objects appearing larger than they would out of the water. The magnification is about 25%, so that a 40 cm long fish will appear to be 50 cm in length. Exactly the same mechanism occurs when we look down into very clear water from above the surface (light coming from the fish diverges as it passes from the water into the air) so that fish we see in the water appear 25% larger than they really are.

We have several mechanisms for helping us determine how far away an object is. One is the apparent size of the object. We know how large our dive buddy is on the surface at various distances (they appear smaller when they are farther assay). When we look at our buddy underwater, we notice how large they appear to be and use our experience on land to determine how far away they are. The problem is that underwater they appear 25% larger than they do in the air and thus we think that they are 25% closer than they actually are. If we reach out to touch them (or anything else) we may find that our arm is too short!

The bottom line is that underwater things generally appear to be 25% larger and 25% closer than they really are. Therefore, divers have a much better excuse for telling stories about how large the fish was than their above water friends!

The eye is really just a complex, living camera. The main parts of the eye are the cornea, the iris, the lens, and the retina. The cornea is clear and has no blood supply It protects the eye but its main function is to focus the light that enters the eye on the retina. A diopter is a measure of the power of a lens and the power of the cornea is equivalent to a +43 diopter lens. It is composed of five layers but for simplicity can be thought of as having a thin layer on the surface (epithelium) and a body composed of flat stromal cells. The epithelium has many nerve endings and therefore, when we get a hair or some other foreign body in the eye and scratch the cornea, it hurts a great deal!

After passing through the cornea, light crosses the anterior chamber and then goes through the opening in the iris called the pupil. The iris is a muscle and forms the colored part of the eye so that when we say someone has blue eyes or brown eyes, we are really saying that their iris is blue or brown. The iris functions the same as the aperture in a camera; it leaves a large opening when the light is dim and a small opening when the light is bright. This controls the amount of light that enters the posterior part of the eye.

When we are excited or frightened the pupil dilates, allowing more light into the eye and allowing us to see more clearly. Men consider women more beautiful if they have larger pupils (they assume the woman is excited to see them!). Women in ancient Egypt used to rub juice from the belladonna plant (contains the drug atropine) in their eyes to dilate the pupils to make themselves appear more beautiful!

Light has to be ‘bent’ just the right amount so that it is focused on the retina for us to see clearly. Light from objects that are far away has to be ‘bent’ less than light from near objects. The purpose of the lens is to change the refractive power of the eye to accommodate for objects at different distances. As the ciliary muscles contract, they release the tension on the suspension ligament of the lens and the natural elasticity of the lens capsule causes it to assume a more rounded

shape. In a normal eye light from distant objects is focused on the retina with the biliary muscles relaxed. To focus the light from near objects, the colliery muscles contract and the lens becomes rounder, increasing the focusing power of the eye.

When light hits the retina, a series of chemical changes occur which generate an electrical signal that is transmitted to the brain and through a very complex process translated into the images we see. Now that we have a basic understanding of hose the eye works, what happens when we try and see underwater?