Water in a drinking glass appears clear and colorless, yet vast natural bodies of water display a distinct blue or blue-green hue. This observation often leads to the incorrect assumption that water simply reflects the color of the sky. The true color of water is an intrinsic property of the water molecule itself, revealed only when light interacts with a sufficient volume of the liquid. The visible color of any body of water is a complex interplay between this inherent hue and the presence of dissolved or suspended substances.
The Colorless Illusion
The appearance of transparency in a small volume of water, such as a glass, is a direct result of the short distance light travels through it. Light is absorbed by water molecules, but this absorption is extremely weak across the visible spectrum. A glass of water contains too few H₂O molecules along the light’s path to absorb a detectable amount of light before it reaches the eye. This clarity is essentially an optical illusion related to scale. Only when the light path length is significantly increased, such as in a deep pool or a long tube of purified water, does the true color begin to emerge.
The Intrinsic Blue
Pure water possesses a faint but definite intrinsic blue color, rooted in the molecular structure of H₂O. When white sunlight penetrates a large volume of water, the water molecules selectively absorb the longer wavelengths of the visible light spectrum. Specifically, red, orange, and yellow light waves are absorbed more efficiently than the shorter blue and violet waves.
This selective absorption is due to the vibrational energy states of the water molecule’s oxygen-hydrogen bonds. The energy of red light photons matches the energy required to excite these molecular vibrations, causing the light to be absorbed. Because the red end of the spectrum is largely removed, the remaining light scattered back to the observer is predominantly blue. This intrinsic blue is distinct from the blue of the sky, which is caused by Rayleigh scattering of light by air molecules.
External Influences on Water Color
While the deep blue of the open ocean is mostly the intrinsic color of pure water, the color of many other natural bodies is often modified by suspended and dissolved materials. The most common cause of green water is the presence of phytoplankton, microscopic plant-like organisms containing chlorophyll. Chlorophyll efficiently absorbs blue and red light but reflects green light, causing the water to take on a noticeable green hue.
Dissolved organic matter (CDOM) also plays a significant role. CDOM often originates from the decay of terrestrial vegetation, leaching tannins into the water and causing it to appear yellow or brown. High concentrations of silt, clay, or sediment suspended in the water can also drastically alter the color, leading to turbid, muddy, brown, or milky gray appearances. In rare, highly concentrated algal blooms, the water can even take on a reddish or brownish-red color, commonly referred to as a “red tide.”
Sky Reflection Versus True Color
A common misconception is that the ocean is blue only because it mirrors the sky. While the water’s surface does act like a mirror, reflecting the color of the sky, this effect is a minor contributor to the deep blue color we observe. The reflection is most noticeable at low viewing angles or when the water surface is extremely calm. The primary evidence against the reflection theory is the observation that large bodies of water retain their blue color even on cloudy or overcast days, when the sky is gray. Furthermore, water viewed from space or in deep, covered swimming pools still exhibits its characteristic blue tone.