Light and Sound
Light and sound waves behave very differently in water than in air. Most light wavelengths are quickly absorbed by water, a fact that both explains why the sea is blue and why ocean life is concentrated near its surface—almost the entire marine food chain relies on light energy driving plant growth. Sound, in contrast, travels better in water, a fact exploited by animals such as dolphins.
Light Waves in the Ocean
White light, such as sunlight, contains a mixture of light wavelengths, ranging from long (red) to short (violet). Ocean water strongly absorbs red, orange, and yellow light, so only some blue and a little green and violet light reach beyond a depth of about 130 ft (40 m). At 300 ft (90 m), most of even the blue light (the most penetrating) has been absorbed, while below 650 ft (200 m), the only light comes from bioluminescent organisms, which produce their own light. Because they rely on light to photosynthesize, phytoplankton are restricted to the upper layers of the ocean, and this in turn affects the distribution of other marine organisms. Intriguingly, many bright red animals live at depths that are devoid of red light: their color provides camouflage, since they appear black.
Light and Sea Colors
Seawater has no intrinsic color—a glass of seawater is transparent. But on a clear, sunny day, the sea usually looks blue or turquoise. In part, this is due to the sea surface reflecting the sky, but the main reason is that most of the light coming off the surface has already penetrated it and been reflected back by particles in the water or by the sea bed. During its journey through the water, most of the light is absorbed, except for some blue and green light, which are the colors seen. Other factors can modify the sea’s color. In windy weather, the surface becomes flecked with white, caused by trapped bubbles of air, which reflect most of the light that hits them. Rain interferes with seawater’s light-transmitting properties, so rainy, overcast weather generally produces dark, gray-green seas. Occasionally, living organisms, such as “blooms” of plankton, can turn patches of the sea vivid colors.
Sound Under Water
The oceans are noisier than might be imagined. Sources of sound include ships, submarines, earthquakes, underwater landslides, and the sounds of icebergs breaking off glaciers and ice shelves. In addition, by transmitting sound waves or bouncing them off underwater objects (echolocation) whales and dolphins use sound for navigation, hunting, and communication. Sound waves travel faster and farther underwater than they do in air. Their speed underwater is about 5,000 ft (1,500 m) per second and is increased by a rise in the pressure (depth) of the water and decreased by a drop in temperature. Combining these two effects, in most ocean regions, there is a layer of minimum sound velocity at a depth of about 3,300 ft (1,000 m). This layer is called the SOFAR (Sound Fixing and Ranging) channel. The properties of the SOFAR channel are exploited by people using underwater listening devices and, it has been theorized, by animals such as whales and dolphins.
Looking up Seen from underwater, only a part of the surface of the sea appears lit up, while the rest looks dark. This is an effect of the way light waves are bent (refracted) when they enter the sea from the air.