The color blue, a pervasive hue in nature from vast oceans to clear skies, often seems inherent to the objects displaying it. However, the perception of color is not simply an intrinsic property of an object. Instead, it arises from intricate interactions between light, matter, and our visual system, with blue manifesting through several distinct scientific mechanisms.
How Objects Get Their Color
The colors we observe are fundamentally tied to how objects interact with visible light. White light, such as sunlight, is a combination of all colors of the rainbow, each corresponding to a specific wavelength within the electromagnetic spectrum. When this light strikes an object, some wavelengths are absorbed, while others are reflected or transmitted. The particular wavelengths that an object reflects are what our eyes perceive as its color. For instance, a blue object absorbs most wavelengths of light but reflects primarily the blue wavelengths.
Our eyes contain specialized cells called cones that are sensitive to different wavelengths of light, typically categorised as red, green, and blue. When reflected light enters the eye, these cones are stimulated to varying degrees, and the brain processes these signals to create our perception of color.
Blue from Light Scattering
One significant way blue is produced in nature is through a phenomenon called Rayleigh scattering. This occurs when light interacts with particles much smaller than its wavelength, such as the molecules of gases in the Earth’s atmosphere. Shorter wavelengths of light, like blue and violet, are scattered more effectively and intensely than longer wavelengths, such as red and yellow. This preferential scattering of blue light is why the sky appears blue during the day; the blue light is scattered across the atmosphere, reaching our eyes from all directions.
The same principle contributes to the blue appearance of some human irises. Unlike brown eyes which have a high concentration of melanin pigment, blue eyes contain very little melanin in the front layer of the iris. Instead, the blue color results from the scattering of light by the collagen fibers within the iris’s stroma, with the shorter blue wavelengths being scattered and reflected more prominently.
Blue from Pigments
In contrast to scattering, blue can also be created by pigments, which are chemical compounds that selectively absorb certain wavelengths of light and reflect others. A blue pigment appears blue because its chemical structure absorbs most of the red, yellow, and green wavelengths of light, reflecting predominantly the blue ones. This absorption is due to the way electrons within the pigment molecules interact with light energy.
Examples of blue pigments include ultramarine, Prussian blue, and phthalocyanine blue. Ultramarine, historically derived from the mineral lapis lazuli, is a complex sodium silicate containing sulfur, with its intense blue color attributed to unpaired electrons in sulfur radical anions. Prussian blue, synthesized in the 18th century, is an inorganic complex salt. Phthalocyanine blue, a synthetic organic pigment, is highly valued in paints and inks for its vibrant color, lightfastness, and chemical stability.
Blue from Structural Color
A distinct and often striking mechanism for producing blue is structural color. This phenomenon arises not from chemical pigments, but from the interaction of light with microscopic physical structures on a surface. These structures, which can be tiny ridges, layers, or photonic crystals, are typically on the same scale as the wavelength of visible light. They manipulate light through processes like interference and diffraction, causing specific wavelengths to be enhanced or canceled out.
A prime example is the brilliant blue of Morpho butterfly wings. Their wings are covered with tiny scales featuring nano-scaled ridges and layered structures that reflect and diffract blue light waves, while other colors are absorbed by underlying brown pigment. The peacock’s vibrant blue feathers also display structural color, where a nanoscale network of keratin and melanin rods selectively reflects blue light. Unlike pigment-based colors, structural colors can often appear iridescent, meaning their hue changes with the viewing angle.