Why is the sea blue?

mar blava


Water in small quantities, such as a glass or a bottle, is perceived as a transparent liquid; however, when a large quantity of water is observed, as happens in the sea, we notice a blue color that can vary depending on the mass of water we are observing. This phenomenon is due to the fact that water does not absorb equally the electromagnetic radiation of different wavelengths that constitute the white light from the sun.

When sunlight reaches the surface of the water, it absorbs red and infrared radiation much more than the radiation corresponding to the blue range. This unabsorbed blue radiation is reflected by the surface and reaches our eyes, making us perceive the sea blue.

When we are on the beach and we move the water with our hands, we notice that it looks transparent, because the absorption is minimal. If we swim in shallow water the blue colour is lighter, while in deeper water and in open sea we will notice that the blue colour becomes more intense because the absorption is considerable. It is even possible to observe different variations of blue depending on other elements found in the sea that also absorb radiation.

On cloudy days, the sea looks grey, as clouds absorb a significant part of white light, eliminating part of the electromagnetic radiation that reaches the sea.

Inmaculada Pascual Villalobos is a professor at the Department of Optics, Pharmacology, and Anatomy of the University of Alacant.

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