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!
