What is refraction and visual inversion?

Refraction is the tendency of light to bend as it passes from one medium to another, such as from water to air. It occurs when light travels at different speeds through various substances due to different densities.

For example, light travels through air at a faster rate than through water, so when light enters water, it enters at a different angle. Unless, of course, you enter a pure 90 degree angle, like the sun when it is straight over your head.

So for divers to see clearly underwater, we wear masks. Now light must travel from water, glass, and air to reach our eyes. During each pass, the light must bend, mold itself through each medium before we see the object. As a result, objects will appear closer to us in a 4: 3 ratio than their actual size. Which means that an object that is 4 feet away will appear to be only 3 feet away. This is why it is difficult to really determine how far an object is in relation to you as a diver, as explained in the next paragraph.

There is a tendency to underestimate distance due to refraction. At greater distances and under certain conditions, objects may appear further away than this relationship suggests. This phenomenon is called visual inversion, where objects appear further away than they really are. In other words, the visual inversion is just that, an inversion of what you would expect if you saw an object on the ground.

This inversion is depth dependent and appears to be the result of a decrease in brightness and contrast. The deeper you dive, the less light can penetrate the water. So it gets drastically darker the deeper you dive. The same goes for color, the deeper the dip, the less color appears. As a side note, this is why many carry underwater lights to see colors, even if there is a lot of light on land.

It can also result from the absence of visual / distance cues like we have on the ground.

In very cloudy water (low visibility), even relatively close objects tend to look further away.

As a rule of thumb, and in closing, when estimating distances, the closer the object is, the more likely it is to appear closer than it is actually due to refraction. The more cloudy the water, the more likely the nearby object is to appear further away than is due to visual inversion.