Why does the sea appear blue?
The sea appears blue because water absorbs more red and yellow light than blue light.
A glass of water looks colorless, but in fact it is a very light blue. If you have enough water in one place, as in the deep sea, this blue color becomes visible.
Sunlight falling on the sea is made up of white light, which comprises all the colors of the spectrum. Some of the sunlight reflects off the surface of the water; the rest of it passes into the water. The water molecules absorb certain colors or wavelengths of light more than others. The colors they absorb most are red, yellow, and green, and this leaves blue light. Some of this blue light keeps going down into the water, and some is scattered by the water molecules so that it rises from the water and meets your eyes. About 65 percent of the visible light entering the ocean is absorbed within the first 3 feet or so of water; less than 1 percent penetrates as far as about 100 yards, and this is entirely blue light.
But the sea is not always blue. Close to the coast it often looks green, and it is also possible to get red, yellow, and brown seas. These colors depend on the composition of the water and which colors of light the water absorbs or reflects.
Muddying the waters
Close to the coast, there tends to be a lot more sediment (small particles of dirt) in the water because of all the dirt that washes off the land into the sea. This sediment changes the way light is absorbed and reflected; the floating specks of dirt tend to absorb red and blue light the most, leaving green light to reflect back out of the water and make coastal waters appear green.
Various other colors can be seen as the result of tiny plants floating in the water. Known as phytoplankton, these tiny plants use pigments such as chlorophyll to harvest sunlight, and they can stain the water. Green phytoplankton make the water look green, but phytoplankton come in many different colors, depending on the pigment used by each species. Some phytoplankton species use red pigments, and these are responsible for what are called red tides.
The seabed also affects water color if the sea is shallow enough for light to reach and reflect off it. In tropical areas with clear, shallow water, the ocean appears very light blue because of the color of the white sandy bottom mixing with the blue water.
Water covers 71 percent of the earth’s surface, and this is why the earth appears blue from space.
