Why does sea water change color

Have you ever wondered why some days the sea seems greener or bluer? Have you seen that it sometimes has a reddish or brownish color? Or why is it sometimes lighter or darker? You have undoubtedly already noticed that the color of the sea water changes, but it does not do so capriciously, nor does the color of the water change as such.

White light and color changes in sea water

Before we dive into the main topic of this article, there are a couple of main ideas that you should know to make it easier for you to understand what happens so that we see color changes in seawater .

The light emitted by a star, in our case the Sun, is known as ” white light “. This white light is actually an overlay of rays of different colored lights. These lights have their own characteristics, such as their wavelength and frequency.

Another concept that we must understand, along with that of white light, is the phenomenon of light scattering . This phenomenon consists in that when a ray of white light passes through a transparent medium, as in our case, water, the light is refracted, that is, it changes direction and speed when changing from one medium to another, thus producing that the resulting beam of white light shows the colors of which it is composed. Thus, as we see, white light makes it possible for us to see different colors in sea and ocean water.

Is sea water really transparent?

Surely if you stop to look at the water you would end up thinking that the water is transparent in color, and the uncertainty would end. The truth is that water looks transparent in small amounts, like in a glass. However, have you not gone to the sea or the ocean and have you seen that the water is blue and that as you go in to bathe the water seems transparent to you? Or have you taken it in your hands and again, has it lost its color?

The reason why we see water colored blue, generally, resides in the phenomena discussed in the previous section, especially the way in which different objects and media absorb white light . Thus, to be able to appreciate the true color of the water we have to observe it in large quantities, as in the sea.

Now, having already made the previous points clear, we can fully delve into what the color of the water is, or why the sea water changes color. The truth is that the color of the sea depends on many factors such as the color of the seabed or its depth , and not only on the color of the sky.as was believed for many years. It is true that the sea looks like a reflection of the sky, but its color is influenced by other things. In the case of the reflection of the sky, what really occurs is the phenomenon of the absorption of white light from the Sun. For example, on cloudy days the sea is seen in gray colors as if it were indeed a mirror of the sky. What happens in this case is that the clouds have previously absorbed a large part of the white light from the Sun.

We know that water itself is transparent. Water molecules absorb a specific part of white light. To a greater extent, they absorb light rays with longer wavelengths, that is, rays of red, yellow, orange and green colors. By absorbing them and letting only shorter wavelength rays pass through, such as blue ones, they can penetrate further into the sea and give deeper waters a dark blue hue. In the sea, when the depth is very high, the rays of light do not reach the seabed, so they are not reflected and are more diffused, making these deep waters acquire a dark blue hue.

However, as we mentioned, there are other factors that influence changes in the color of the sea . In relation to the absorption of white light, the color of the sea changes depending on the hours of the day and the seasons of the year. We know that the way the Sun’s rays strike the Earth’s surface varies. In this case, these variations in the color of the sea are due to the fact that the absorption of light rays varies, among others, depending on their obliquity.

Particles found in suspension in its waters also influence the color of the sea, as they vary the ability of water to absorb light and increase its dispersion. Particles of sand or mud and organic debris particles can turn water towards more green, yellow or brown.

Finally, there are also biological factors such as phytoplankton or microalgae proliferations (blooms), which cause ” red tides “, which stain the sea waters due to the pigments they possess. In the case of the first, the phytoplankton absorb red and blue light, and in this case, it reflects green light, so the waters in which the phytoplankton are in large quantities become greener. This phytoplankton is a great indicator of the health of the planet and the level of contamination, as it proliferates when changes occur in its environment such as changes in temperature or contamination due to excess nutrients.

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