Neither the sky nor the chlorine: the reason why pool water is blue

If you thought it was just because of the reflection of the sky or the chemicals in the water, you are not alone. Many are sure they know exactly why the water in a public or private pool looks a spectacular light blue. However, a new professional guide published by Gres Aragón, the leading European ceramic and cladding company from Spain, shatters the myths and reveals the surprising physics behind the familiar look.

The myth broken: water has a color of its own

According to the guide, the thought that water is completely transparent and only shows the sky is a common mistake. In fact, water has its own natural color resulting from a physical phenomenon called Selective Absorption.

The explanation lies in the nature of sunlight, which is composed of electromagnetic waves of different colors. When white light rays penetrate the pool, the water molecules act as a natural filter: they quickly absorb the long wavelengths in the light spectrum — the red, orange, and yellow colors. In contrast, the short wavelengths — mainly blue and green — are not absorbed. They are reflected, scattered in the water, and reach our eyes directly. This is the scientific reason why, above a certain volume, clear water will always look bluish.

The "Optical Triangle": how the exact shade is determined

The final look of the pool is not accidental, but the result of an optical formula consisting of three factors working together:

• The sky: presents the source of light and reflection. A clear sky provides strong light and powerful blue reflection, while a cloudy sky reduces the intensity of the color.

• The thickness of the water layer: the water acts as a filter. The deeper the pool, the more the light passes through a thick layer of water, absorbing more warm tones and intensifying the blue hue. This is why the original tile is visible on the shallow steps, and as it gets deeper, the water becomes darker and more pronounced.

• The color of the tile in the background: the bottom surface from which the light is reflected back up through the water.

What does the tile color say about your water?

At Gres Aragón, they explain how choosing the coating color completely changes the visual perception of the eye:

• White tiles: these reflect all the light, so when it passes through the natural water filter, a clear, bright, and clean sky-blue color is obtained.

• Sand or beige tiles: the physical encounter between the yellow of the base and the natural blue of the water creates an illusion of a turquoise shade for the eye.

• Gray or black tiles: these absorb most of the light and give the water a deep blue shade, while creating a mirror effect for the reflection of the surrounding buildings and vegetation.