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Why is water clear but blue?

Why is water clear but blue?

Water appears colorless and transparent in small quantities, but takes on a noticeable blue tint in larger bodies like oceans and lakes. This phenomenon has fascinated scientists and observers for centuries. The blue color is caused by selective absorption and scattering of sunlight as it passes through water.

The Science of Light and Color

To understand why water is blue, we first need to review some basic principles of light and color:

  • Sunlight is made up of a spectrum of light waves of different wavelengths. Wavelengths are measured in nanometers (nm).
  • The visible spectrum ranges from violet light at around 400 nm to red light at around 700 nm.
  • Water molecules absorb light waves in the red end of the spectrum (600-700 nm).
  • Blue and violet light (400-500 nm) are scattered more than other wavelengths.

Absorption by Water Molecules

The water molecule is made up of one oxygen atom bonded to two hydrogen atoms (H2O). This molecular structure allows water to absorb certain wavelengths of light in the visible spectrum, specifically in the red end.

As sunlight enters a body of water, water molecules interact with the light waves. The oxygen-hydrogen bonds absorb light energy at the red end of the spectrum (around 600-700 nm). This absorption removes red wavelengths and shifts the visible color toward blue and violet.

Scattering Effects

In addition to molecular absorption, the blueness of water is enhanced by preferential scattering of blue and violet light waves. Scattering occurs when light waves interact with particles in the water and change direction.

Shorter wavelengths of blue and violet light are scattered more easily than longer red wavelengths. This scattering diffuses the blue colors throughout the water, while red light tends to travel straight through. This further reduces the red hues at depth.

Light Wavelength Absorption Scattering
Red – 700 nm High absorption Low scattering
Blue – 470 nm Low absorption High scattering

This table shows that blue light is less absorbed and more scattered than red light in water, leaving blue as the predominant color.

Factors Affecting Water Color

Several factors influence the intensity and shade of blue in water bodies:

  • Depth – Deeper water appears darker blue due to increased absorption of light.
  • Density – Denser water scatters light more, increasing blueness.
  • Particle content – More sediments or organisms can scatter light and add brown, green, or gray hues.
  • Viewing angle – Looking through more water depth increases blue color.

In small amounts, these factors have minimal effect, so water maintains its clear, colorless appearance. But in oceans and lakes, the cumulative effects make the blue color clearly evident.

Examples of Blue Water

Here are some examples that illustrate the blueness of water bodies:

  • The oceans appear deep blue when viewed from space, airplanes, or mountainsides.
  • Tropical seas often appear bright turquoise due to shallow depths and white sand bottom.
  • Glacial lakes reflect rich blue hues due to high water density and few particles.
  • Swimming pools appear light blue when looking across the length rather than down into them.

Even in your drinking glass, the blue tint becomes visible when light passes through several inches of water. So even ordinary tap water has traces of blue if you look for it!

Water Clarity

Despite the blue color, water still appears mostly clear for several reasons:

  • Absorption only affects red hues, leaving most other wavelengths.
  • Scattering is fairly uniform in all directions.
  • The blue shade is subtle without high concentrations.

In small volumes, this faint blue tint is hard to notice against white surfaces. So a glass of water maintains its see-through appearance. But in larger depths and quantities, the blue becomes more apparent to our eyes.

Unique Properties of Water

The blueness of water is directly linked to its unique molecular properties that control absorption and scattering:

  • Water molecules are polar, with uneven charge distribution.
  • They form hydrogen bonds with each other.
  • The bonds vibrate at frequencies matching red wavelengths.
  • This allows for selective absorption in the red part of the spectrum.

No other common, colorless liquid shares these traits. Thus, water stands alone in its ability to absorb red and transmit blue under normal conditions. The blueness is a signal of water’s molecular complexity.


In summary, water owes its intrinsic bluish color to the absorption of red light and scattering of blue wavelengths. This is caused by the hydrogen bonds and asymmetric shape of the water molecule. Many factors control the intensity of the blue shade, but it is always present subtly if you look for it. Next time you fill a glass of water, take a moment to appreciate the pale blue tint as you drink it.