It turns out that seawater, like ordinary water, is colorless and transparent, and the color of the ocean is determined by the optical properties of seawater and the suspended matter contained in seawater, the depth of seawater, the characteristics of clouds and other factors. As we all know, sunlight is composed of red, orange, yellow, green, cyan, blue, purple seven colors, these seven colors of light, wavelengths are different, from red light to violet light, the wavelength gradually shortened, long wave penetration ability is the strongest, the most easy to be absorbed by water molecules, short wave penetration ability is weak, easy to reflect and scatter. The degree of absorption, reflection and scattering of light of different wavelengths by seawater is also different. The long red, orange, and yellow light waves are gradually absorbed with the increase of ocean depth after hitting the seawater. Generally speaking, in oceans with water depths of more than 100 meters, most of these three wavelengths of light can be absorbed by seawater, and the temperature of seawater can also be raised. The shorter wavelength of blue light and violet light will be strongly scattered and reflected when it encounters purer seawater molecules, so the ocean seen by people is blue or dark blue. Due to the increase of suspended matter, larger particles, weak absorption of green light, and strong scattering, seawater near the shore is mostly light blue or green.
Violet light has the shortest wavelength and the strongest reflection, why doesn't seawater appear purple? Scientific experiments have proved that the original human eyes have a certain bias, human eyes are very weak in the ability to feel purple light, plus its radiation intensity is not as good as blue light, so it is very insensitive to the purple reflected by seawater, so it is blind, on the contrary, human eyes are more sensitive to blue and green light.
The vast majority of the ocean is blue, and if there is more suspended matter in the seawater, or other reasons, the color of the sea is no longer blue. For example, China's Yellow Sea, which is the estuary of the ancient Yellow River, a large amount of sediment carried by the Yellow River flows into the sea, "dyeing" the blue seawater. Although the current Yellow River pours into the Bohai Sea instead, the northern part of the Yellow Sea is connected to the Bohai Sea through the Bohai Strait, plus it has to withstand the water injected by the Huaihe River, Guanhe and other rivers, so the sea surface still shows a light yellow color.
In the northwest of the Indian Ocean, the Red Sea between Asia and Africa is the hottest sea in the world, and a reddish-brown seaweed grows in the sea, because this seaweed grows in large numbers all year round, dyeing the sea surface a red, hence the name.
The Gulf of California in the northeast of the Pacific Ocean, with blood-red seaweed colonies in the south, and a large amount of red soil brought by the Colorado River during the rainy season in the north, the sea is reddish-brown, known as the Vermilion Sea.
The White Sea is the marginal sea of the Arctic Ocean, which is deep in the interior of northwestern Russia and runs through the White Sea by the Arctic Circle. Due to the high latitude, cold climate, ice and snow all year round, and the low organic content of the White Sea, the sea water is white, hence the name "White Sea".
On the surface of the Black Sea, there are fresh water inflows such as the Don, Dnieper, and Doehe rivers, and the density is small; The deep layers of the Black Sea are high salt water from the Mediterranean Sea and are dense. A density leap layer is formed between the upper and lower seawaters, which seriously hinders the water exchange between the upper and lower water layers. The Black Sea is exchanged with the Mediterranean Sea via the Bosphorus and Dardanelles. Since the straits are narrow and shallow, which greatly limits the exchange of water between the Black Sea and the Mediterranean, the deep Black Sea lacks oxygen, the filth secreted by organisms in the upper seawater and the corpses after death sink to the depths to rot and smell, and a large amount of sludge turbid water makes the ocean black. In addition, the Black Sea region is often rainy and stormy, which adds to the feeling of blackness.
Red tides can also cause the color of the sea to appear abnormal. Red tide is a phenomenon in which plankton suddenly multiplies and clumps together due to the sudden and sharp reproduction of plankton in local sea areas. The color of red tide is varied and it mainly depends on what kind of marine plankton causes red tide. Red tides caused by noctilucent insects are pink or brick-red, red tides caused by some diflagellates are green or brown, and some diatom red tides are yellowish brown or reddish-brown.
In addition, because the sun sometimes disappears into the clouds, sometimes emits light through the clouds, the color of the ocean changes. The color of the ocean also depends on the height of the sun from the horizon.