The ocean often appears blue because sunlight entering seawater is selectively absorbed and scattered, while the light that returns from the water to an observer contains a greater relative contribution from blue wavelengths under many conditions. Water is not perfectly transparent to visible light. Its molecules absorb electromagnetic energy more strongly at longer visible wavelengths, especially towards the red end of the spectrum, while shorter blue wavelengths can penetrate farther before being absorbed.
At the same time, microscopic fluctuations in water density and suspended particles scatter some light. The combination of wavelength dependent absorption and scattering gives deep, relatively clear water its characteristic blue appearance. The effect is fundamentally different from the ocean simply reflecting the blue sky, although reflected sky light can contribute to the observed appearance. Water can look blue even when viewed from conditions where the sky is not strongly blue, and the spectral properties of water itself are therefore essential to the explanation.
Water molecules have vibrational and rotational transitions that produce stronger absorption at particular infrared wavelengths and weaker but still significant absorption extending into the visible spectrum. In the visible region, absorption is generally stronger toward red than blue, meaning that red light is attenuated more rapidly as it travels through sufficiently deep water. Blue and blue green wavelengths can therefore survive to greater depths.
In very pure water, the intrinsic optical properties of water become especially important because there are fewer suspended particles or dissolved substances to alter the spectrum. The resulting colour depends on the absorption coefficient and scattering coefficient of the water, which vary with wavelength. Ocean optics therefore treats water as an optical medium whose behaviour can be described quantitatively using radiative transfer, absorption, scattering and refractive index rather than simply describing the water as transparent.
Scattering is also important because water is not optically homogeneous at microscopic scales. Molecules and small structures within the water can redirect some incident light, and suspended particles can contribute substantially when present. In clear open ocean water, molecular scattering can help return blue wavelengths towards the surface and an observer. In coastal or biologically productive waters, phytoplankton, sediment, dissolved organic matter and other substances can change the colour dramatically.
Phytoplankton containing pigments such as chlorophyll absorb particular wavelengths and can make water appear green. Sediment can produce brown, yellow or grey water by strongly scattering light. Dissolved organic material can produce darker or yellowish tones. Ocean colour is therefore an optical measurement of the combined effects of water molecules, particles, organisms and dissolved substances rather than a single universal colour intrinsic to every body of seawater.
The sky can contribute to ocean colour because the water surface reflects part of the incident illumination. The amount of reflection depends on the angle of the surface and the direction of observation, and specular reflection can make the water mirror the sky strongly under particular geometries. However, reflection alone does not explain why large volumes of clear water have a blue appearance.
If pure water were optically neutral throughout the visible spectrum, reflected sky light would be a much more dominant explanation. The fact that water preferentially removes longer visible wavelengths as light travels through it means that water itself has an important spectral signature. This distinction is especially evident in deep water, where the blue colour can be strong even when the observer is looking into the water rather than simply at a mirror like surface.
Depth strongly affects ocean colour because the longer the optical path through water, the greater the cumulative absorption. A shallow layer of water may appear nearly colourless because insufficient distance is available for strong wavelength selective absorption to occur. As depth increases, red and orange wavelengths are increasingly removed, leaving proportionally more blue or blue green light. In very deep water, most visible wavelengths are eventually absorbed, and the water can appear extremely dark.
This is why underwater environments become progressively darker with depth and why deep ocean regions receive little sunlight. The penetration depth varies by wavelength and water clarity, so different colours disappear at different rates. Oceanographers use these optical properties to study underwater light fields, photosynthesis, biological productivity and the movement of materials through the marine environment.
Human perception also influences descriptions of ocean colour. The visual system responds to the spectrum of light reaching the eye, and surrounding colours, illumination conditions and adaptation affect perceived hue. A clear tropical ocean may appear vivid turquoise because shallow sand or carbonate bottoms reflect light through relatively clear water, while a deep offshore region may appear dark blue.
Green coastal waters can result from phytoplankton or suspended material, while glacially influenced water may appear turquoise because fine mineral particles scatter light efficiently. Thus, the phrase blue ocean describes a common appearance rather than a fixed physical colour. Instruments such as spectroradiometers can measure the wavelength dependent reflectance and radiance of water more precisely than visual observation.
Ocean colour has major scientific applications. Satellites can measure reflected light from the ocean surface and use spectral information to estimate quantities such as chlorophyll concentration, suspended sediment and certain characteristics of dissolved material. These measurements support studies of marine ecosystems, algal blooms, sediment transport, water quality and large scale ocean processes. Remote sensing algorithms must account for atmospheric scattering because satellites observe light after it has passed through the atmosphere.
They also have to distinguish water leaving radiance from reflected sky and other contributions. Ocean colour science therefore combines optics, atmospheric physics, biology, chemistry, remote sensing and computational modelling. The apparent colour of the ocean can consequently provide information about the physical and biological state of the marine environment.
The common claim that the ocean is blue simply because it reflects the blue sky is incomplete. Reflection contributes to observed water colour, especially at particular viewing angles, but the intrinsic absorption and scattering properties of water provide the fundamental explanation for the blue tendency of deep clear water. The ocean is also not uniformly blue because depth, particles, dissolved substances, organisms, bottom sediments, waves, illumination and viewing geometry all affect the spectrum reaching an observer.
Understanding ocean colour therefore requires considering the complete optical path from sunlight through the atmosphere, into and through the water, back towards the surface and finally into the eye or detector. The familiar blue appearance is an emergent result of wavelength dependent absorption, scattering and reflection rather than a simple pigment or surface property.
