Light and Color Underwater

Water absorbs color

Sunlight is a mix of colors. As it passes through water, each color is absorbed at a different rate. Long wavelengths go first: red fades within the first few metres, orange and yellow follow over the next 10 to 20 metres, and green and blue reach deepest. That is why the deep sea looks blue and why a red fish can look grey or brown at 20 m (66 ft).

The depth at which each color disappears depends on the water. Clear tropical water lets light travel further than green coastal water full of plankton or silt. In any water, the distance light travels counts too, not just depth: a subject 3 m away horizontally loses color as well.

Bringing color back

Artificial light restores the colors that water has absorbed, because it only travels a short distance to the subject and back. A dive light shows the true reds and oranges of coral, sponges and fish, even in the middle of the day. Underwater photographers use strobes or video lights for the same reason, and red or magenta filters to shift the color balance in shallow water. On a night dive your light is the only light, which is part of why reefs look so vivid after dark.

Refraction: bigger and closer

Light bends when it crosses from water into the air inside your mask. This bending, called refraction, makes objects look about a third larger and about a quarter closer than they really are. A fish that seems an arm's length away is often further. New divers often reach for things and miss, and the effect also makes it easy to misjudge how close you are to the reef. Practice teaches your brain to adjust.

Visibility

Visibility is how far you can see horizontally underwater. It depends on what's suspended in the water: plankton, sediment stirred up by waves or fins, and runoff after rain. Visibility can range from less than a metre in a silty lake to more than 30 metres in clear open ocean. Poor fin technique can ruin visibility for everyone behind you, which is one reason good neutral buoyancy matters.

Sound underwater

Sound travels about four times faster in water than in air, roughly 1,500 metres per second compared with about 343 in air. Your brain locates sounds on land by the tiny time difference between them reaching each ear. Underwater that difference is too small to notice, so a boat engine or a tank banger seems to come from all directions at once. Never assume you know where a boat is from its sound alone. This is also why divers rely on hand signals and simple noisemakers rather than speech.

Frequently asked questions