Blue is one of the most commonly occurring colors in nature. From expansive blue skies and oceans to the diverse blue hues found across plant and animal species, blue is ubiquitous in the natural world. But what causes things to appear blue? In this article, we’ll explore some of the key sources of blue coloration in the environment and the science behind them. Understanding what makes things blue can give us a deeper appreciation of both the optical physics and biological processes behind nature’s palette.
Rayleigh scattering: Why the sky is blue
Perhaps the most familiar blue sight is the daytime sky. On a clear day, the sky takes on a rich blue tone. This blue color is caused by a phenomenon called Rayleigh scattering. Here’s how it works:
As sunlight enters the atmosphere, it collides with gas molecules in the air. These molecules scatter the shorter wavelengths of sunlight more than longer wavelengths. Violet and blue light have shorter wavelengths, so more of this light gets scattered. The scattered violet and blue light spreads out across the whole sky, giving it that blue appearance.
Meanwhile, the longer wavelengths like red and orange aren’t scattered as much. Those pass directly through the atmosphere. At sunset and sunrise, the sun’s light has to pass through more atmosphere to reach our eyes. More of the longer wavelengths get scattered or absorbed, leaving more of the shorter blue light to dominate the sky’s color.
Structural color in animals
Many animals also derive their blue coloration not from pigments, but from microscopic structures in their feathers, scales, or exoskeletons. These structures reflect and scatter light to produce striking blues.
| Animal | Mechanism of Structural Blue |
|---|---|
| Peacocks | Their feathers contain nanoscale arrays of melanin rods that reflect blue and green light. |
| Morpho butterflies | Scales on their wings have multilayered nanostructures that refract light. |
| Blue tang fish | Mirror-like platelet cells called iridophores in their skin backscatter blue light. |
These anatomical tricks allow animals to generate dazzling blue tones without needing blue pigments. Mimicking these structures has also given engineers ideas for manufacturing vivid synthetic colors.
Pigment-based blues
While structural colors account for many blue sights in nature, pigment molecules remain essential for producing blue coloration. Here are some examples of key blue pigments found in the natural world:
Anthocyanins
These water-soluble plant pigments are responsible for the blues, purples, and reds in many flowers, fruits, and vegetables. Foods like blueberries, concord grapes, and purple cabbage contain anthocyanins. The pigments help attract pollinators and protect plants from excess light. Anthocyanins’ color depends on the surrounding pH, taking on red hues in acidic conditions and bluer tones in alkaline conditions.
Psittacofulvins
Found in parrots, this family of pigments imparts the bright blues, greens and yellows in parrot feathers. The blue psittacofulvins possess highly conjugated molecular structures that selectively reflect blue light. By blending blue, green and yellow psittacofulvins in different ratios, parrots can produce a riotous mix of hues.
Billiverdin
This pigment occurs in some fish and invertebrates like lobsters. Unusually, it’s derived from the breakdown of heme proteins, unlike other pigments that are synthesized from various vitamins, amino acids or other compounds. The resulting blue coloration serves as camouflage in blue-hued marine environments.
Carotenoids
Carotenoids are a familiar group of plant pigments including lycopene, beta-carotene and zeaxanthin. While known for producing red, orange and yellow hues, carotenoids can also appear blue when combined with structural coloring. This is seen in the azure mountain bluebird, where blue carotenoids in the feathers combine with structural reflectors to give an intense blue color.
Light scattering by cellular structures
Some blue coloration in plants and fungi stems from light scattering by internal cell structures. The anthocyanin pigments in hydrangea flowers, for instance, appear blue because colorless cell vacuoles and sulfur granules scatter the light. Blue hues also develop in bruises for similar reasons, as hemoglobin breakdown products scatter light.
Rayleigh scattering in water
When sunlight penetrates into clear ocean or lake waters, preferential scattering of short blue wavelengths gives water its archetypal blue color. This is the same Rayleigh scattering process that makes the sky blue, but it now occurs as sunlight passes through water rather than air molecules. The more pure the water, the further blue light penetrates before being absorbed, creating a deeper blue hue.
Selective absorption by water
In extremely deep or very pure waters, blue coloration can also occur due to selective absorption of longer wavelengths. As all sunlight besides blue light gets absorbed in deep waters, only the blue remains to be observed. This typically only happens at depths greater than 200 meters, beyond where most light penetration occurs. But it can contribute to the deep blue appearance of some extremely clear lakes.
Tyndall Scattering
Sometimes blue is produced by Tyndall scattering, the scattering of light off larger colloidal particles or suspended solids rather than individual molecules. Glacial meltwaters often have a blue tint because rock flour particles in the water preferentially scatter blue wavelengths. Tyndall scattering also explains the bluish hue seen when water vapor condenses into the tiny droplets of fog or clouds.
Fluorescence
Finally, some blue coloration arises from fluorescence, where materials absorb UV light and re-emit it at longer, visible blue wavelengths. Some minerals like calcite glow blue under UV light because of fluorescence. And coral reefs can take on a spectacular blue hue due to fluorescent proteins in the corals themselves. When illuminated by the UV rays of the sun, these proteins shine an intense blue that stands out against the darker waters.
Conclusion
From the vast blue ocean to the tiniest blue morpho butterfly, blue coloration arises in nature through a variety of processes. Scattering, refraction, reflection, absorption and fluorescence all contribute to the prevalence of blue shades across animate and inanimate objects. Understanding the optical origins of blue in the natural world gives us insight into both the physical universe and the amazing adaptations of life on Earth. Whether gazing into an alpine lake or admiring a bird’s feathers, we can appreciate anew the visual wonders made possible by the physical properties of light.
