Have you ever wondered why the water in your glass is perfectly clear, but the vast expanse of the ocean appears in stunning shades of blue or green? This everyday mystery has a precise scientific explanation, rooted in the behaviour of light, and is not merely a trick of the eye. Researchers have moved beyond guesswork to provide measured, peer-reviewed answers.
The Physics Behind the Blue: Absorption is Key
A pivotal study published in the journal Applied Optics provides the core answer. Scientists conducted careful measurements of how water interacts with light across the visible spectrum. Their finding was clear: water absorbs longer wavelengths of light, like red and orange, much more strongly than shorter wavelengths, such as blue and violet. This differential absorption is minimal over short distances but becomes dramatically evident when light travels through metres of water.
This is why a glass or a small bowl of water seems colourless. The distance the light travels from one side of the glass to the other is too tiny for our eyes to detect the slight loss of red light. The water effectively "hides" its true optical behaviour. However, when you look at the deep sea, light penetrates down and back through a significant volume. Over this long journey, the red wavelengths are progressively "eaten up," leaving the surviving blue light to dominate what reflects back to our eyes. The sea doesn't add blue colour; it selectively removes red.
How Nature Paints the Water: Algae, Soil, and Reflections
While the basic physics explains the blue, nature adds countless variations to the palette. Pure water is rare in outdoor environments. Various elements interact with light to create the diverse colours we see in different water bodies.
Particles and Life: After heavy rains, rivers can turn brown or muddy as they carry soil and sediment. During warm periods, algal blooms infuse water with green pigments that reflect green light. In shallow coastal areas with white sandy bottoms, the sea often takes on a brilliant turquoise hue because the sand reflects additional light back to the surface.
Sky and Depth: The Final Influences
Reflection from the sky plays a secondary but noticeable role. On a calm, sunny day, a water body can act as a mirror, reflecting the sky's blue and enhancing its own blue appearance. However, this is not the primary reason for water's colour, as evidenced by the fact that oceans and lakes still retain a blue-green tint even on cloudy days. The fundamental absorption process documented in the study continues underneath any surface reflection.
Ultimately, the colour of any water body—from the deep indigo of the open ocean to the milky blue of a glacial lake—is a complex signature. It tells a story defined by physics (light absorption), depth, the suspended particles within it, and the nature of the floor below. A simple glass of tap water keeps this secret, revealing nothing. But the sea, with its vast scale, tells the full and colourful truth.
