Colors in Water

The colors we experience while diving or snorkeling are determined by the way light interacts with water and the organisms living in it. Every underwater scene is shaped by a combination of absorption, scattering, reflection, refraction, and, in some cases, light emission.

During the day, almost all visible light originates from the Sun. As sunlight penetrates the water column, its intensity decreases and its spectral composition changes with depth. The colors we perceive underwater therefore depend not only on the objects themselves but also on the light that remains available to illuminate them.

At night, the situation is entirely different. Without sunlight, the only natural sources of light are the organisms themselves.

Some species produce bioluminescence, generating light through biochemical reactions. Others display fluorescence, in which high-energy blue or ultraviolet light is absorbed and re-emitted at longer wavelengths, typically as green, yellow, orange, or red light. Fluorescence therefore requires an external light source, whereas bioluminescence does not.

Why do Colors Disappear when Diving?

One of the first things every diver notices is that the underwater world becomes increasingly blue with depth. Bright red corals turn dark brown, orange fish lose their vivid colors, and even yellow sponges gradually fade into shades of green and blue.

This phenomenon has a simple physical explanation: water absorbs different colors of light with different efficiencies.

Many people assume that the ocean is blue because it reflects the sky. While surface reflections certainly contribute to its appearance from above, they are not the primary reason. Pure water itself has an intrinsic blue color.

Water absorbs long wavelengths of visible light much more strongly than short wavelengths. Red light disappears first, followed by orange and yellow, while blue light penetrates much deeper into the water column. The remaining blue light is scattered back toward our eyes, giving clear tropical water its characteristic appearance.

The two figures below illustrate how visible light interacts with pure water.

The left figure shows the absorption spectrum of pure water. Lower absorption means that light of a particular wavelength can travel farther before being attenuated. Blue and violet light (approximately 400–450 nm) experience the weakest absorption and therefore penetrate deepest into the ocean, whereas red light above about 650 nm is absorbed much more rapidly (raw data for the figures taken from H. Buiteveld, J. M. H. Hakvoort, M. Donze, "The optical properties of pure water," in SPIE Proceedings on Ocean Optics XII, edited by J. S. Jaffe, 2258, 174-183, (1994)).

The right figure shows how the transmission of sunlight changes with increasing water depth. Even after passing through 20 metres of exceptionally clear water, much of the blue light remains, while most of the red component has already disappeared.

This also explains why ultraviolet radiation penetrates surprisingly deep into clear tropical water. Although its intensity decreases with depth, water provides much less protection from UV radiation than many divers expect. Don't forget your sunscreen—even on diving days.

The Underwater Color Filter

Even a relatively shallow water column acts as a natural color filter.

After only one metre, more than 20% of deep red light has already been absorbed. At 10 metres, less than about 10% of the original red light remains in very clear water. Orange and yellow wavelengths disappear next, leaving an increasingly blue-green light field with depth.

This is why a bright red sponge photographed at 20 metres appears dark brown or almost black unless it is illuminated with an artificial light source. The pigments have not changed—the red light needed to reveal their color is simply no longer available.

What is the Physics behind the Blue Water Color?

The absorption responsible for water's blue color differs fundamentally from that of most pigments. In dyes and biological pigments, color usually arises from electronic transitions, where photons excite electrons within a molecule.

Water behaves differently.

Its weak absorption in the red region of the visible spectrum originates from the low-energy tail of molecular vibrational overtones associated with the O–H bonds. Instead of promoting electrons to higher energy levels, the absorbed light excites molecular vibrations. Although this absorption is relatively weak, it becomes noticeable over distances of several metres or more, giving deep water its unmistakable blue color.

An elegant demonstration of this phenomenon involves heavy water (D₂O), in which hydrogen atoms are replaced by deuterium. Because deuterium is heavier than hydrogen, the vibrational frequencies of the O–D bonds are lower, shifting the absorption bands toward longer wavelengths. As a result, thick layers of heavy water appear almost colorless rather than distinctly blue.

When Water Is Not Blue

The brilliant blue of tropical oceans is characteristic of exceptionally clear water. In many coastal areas, lakes, and estuaries, suspended particles and dissolved organic matter strongly modify the underwater light field.

Phytoplankton and algae contain chlorophyll, which absorbs blue and red light while reflecting green wavelengths, giving productive waters their green appearance. Fine sediment, clay particles, or dissolved organic compounds may produce brown, yellow, or even reddish water.

These colors do not originate from the water molecules themselves but from substances suspended or dissolved within the water.

Why Is the Sky Blue?

Although both the ocean and the sky often appear blue, the underlying physics is different.

Water is blue primarily because it selectively absorbs longer wavelengths of sunlight, allowing blue light to dominate after travelling through the water.

The atmosphere, by contrast, contains gases that scatter short wavelengths much more efficiently than long wavelengths. This process, known as Rayleigh scattering, redirects blue light throughout the sky and gives it its familiar color.

The ocean and the sky may look similar, but the mechanisms responsible for their colors are fundamentally different.