What’s different about being in the Water?
Because water is denser than air, light and sound travel differently under water than they do on the surface. Heat loss is also substantially greater under water than it is on the surface.
Key Points
As you read this section, look for the information that answers this question:
How does the size and distance of objects under water differ from their appearance in air?
- To focus under water, our eyes need an air space in front of them, just as they do on land. This is why we wear dive masks.
- As light passes from the water, through the mask lens and into the air space inside, it bends — a process known as refraction. This creates a magnifying effect.
- Under water, objects appear 25 percent closer and 33% larger than they do on land.
This helps explain why, when you reach out to touch something that appears to be within arms reach; you may find that it is just outside your grasp.
As you read this section, look for the information that answers this question: 
How does depth affect the appearance of colors under water?
- As light passes through water, the water absorbs much of its color.
- Reds are the first to disappear, followed by oranges, yellows, greens and other colors of the spectrum.
- At a depth of 30 m/99 ft, little color remains other than blues, grays, blacks and whites.
- This is why divers frequently carry lights with them, even during the day.
- Dive lights help restore the colors an object would display if closer to the surface.
Suspended particles, known as turbidity, not only reduce visibility, they can also affect color under water. For example:
- The algae found in some bodies of fresh water can give everything a decidedly greenish appearance.
- The more suspended particle matter there is in water, the more rapidly colors are absorbed. This is why there can appear to be less color in shallow, turbid water than there is in deeper water that is clear.
Turbidity’s greatest impact on divers is loss of visibility. Even in the clearest water, you cannot see as far as you can in air. Turbidity can substantially reduce this. Divers express visibility as the distance, in feet or meters, at which an object becomes indiscernible from the surrounding water.
Turbidity can lead to loss of direction and buddy separation. In turbid water, you will need to focus harder on navigation and staying together.
