Why is the Blue Lagoon blue?

The Blue Lagoon is blue because of the way silica—the lagoon’s iconic and most abundant element—reflects visible light when suspended in water. All matter reflects visible light. Depending on the molecular structure of a given entity, it reflects a specific color of visible light. Red paint, for example, is red because it is engineered to reflect only the red wavelengths of visible light. All the other wavelengths—or colors—are absorbed by the paint. To grasp this concept, it is useful to understand the nature of visible light. Visible light is the spectrum of electromagnetic energy that can be perceived by the human eye. It moves at 300,000 meters per second in waves that are approximately the size of a pinhead. Further, the entire color spectrum of visible light is encompassed by white light. This can be shown with the aid of a prism, which allows us to effortlessly split white light into a rainbow of colors, each color corresponding to a specific wavelength.

When visible light encounters matter, its behavior is dependent on the wavelength of the light and the molecular structure of the matter. In general, the various wavelengths are either absorbed or reflected. The reflected colors are what we see. The absorbed colors are invisible.

Returning to silica—a mineral compound consisting of silicon and oxygen with the chemical signature SiO²—it is apparent that when this bioactive mineral is suspended in water, it reflects only the blue wavelengths of visible light. The rest of the colors are absorbed. Thus, the Blue Lagoon is blue.