Nature shows a myriad of colors, but what are colors in the first place? In essence, colors are light of different wavelengths.  

Light is a wave and as such, it has a property called a wavelength, which is the distance between two crests in the wave. Our human eyes can see wavelengths ranging from roughly 380 to 750 nanometers (a nanometer, abbreviated as nm, is 0.000001 millimeter). We perceive these wavelengths as colors, and different wavelengths correspond to different colors. Violet is light of around 400 nm, blue is around 450 nm, green is around 550 nm, and red wavelengths are the longest with roughly 670 nm.

However, you might have heard that some colors do not exist on this spectrum – an example is magenta. How does magenta exist if there is no magenta wavelength? The reason is that color is as much of a physical phenomenon as it is a perceptual experience. Human eyes have 3 types of detectors called cones that perceive blue, green, and red wavelengths. When our cones detect both blue and red wavelengths, our brain adds the two colors and interprets them as a new color that doesn’t exist on the spectrum, magenta.

In this TED-Ed lesson, when we address the question of the rarest color in nature, we are referring to the rarest among the spectral colors—those that exist as wavelengths of light. This subtle but key distinction means that we don’t consider magenta or purple to be equivalent to the violet wavelength at 400 nm, even though sometimes they may look very similar to the eye. So while a study of color is undoubtedly incomplete without acknowledging the perceptual side, we focus on the physical nature of color mainly for simplicity of argument.

In this lesson, we explore how nature finds different ways to make different colors. Some colors are easier to make by absorbing select wavelengths, and others are easier to make by reflecting or scattering select wavelengths. In absorption-based colors, a material absorbs select wavelengths of light, and its final hue consists of the remaining color—the one that wasn’t absorbed. A red pigment is red because the material of the pigment absorbs all the wavelengths that are not red. And in scattering-based color, the material typically contains a nanostructure that reflects some wavelengths and the remaining simply pass through. This type of coloration is known as structural color. Bluebirds are blue because the material of the feathers has a structure that reflects blue and no other colors.

But where do the other colors go? Is the feather transparent to these other colors? Not necessarily. Nature often uses both absorption and structure to fine tune colors and make them vibrant. In bluebird feathers, underneath the structure that reflects blue there are brown pigments that absorb non-blue colors, making the blue look rich and saturated. By using absorption, scattering, or a combination of both, nature can make an immense assortment of beautiful hues.

Researchers have learned extensively from nature’s ability to make colors. In particular, the field of structural colors has grown dramatically in recent years, with researchers understanding key questions about why some colors are rare and discovering new physics in the process. One of such insights is related to the rarity of structural reds. It turns out that, to our knowledge, there are no angle-independent structural reds in nature—that is, structural reds that look red from every angle. There are examples of angle-dependent structural reds (in opal stones for example), which are rare, but exist. But the inexistence of angle-independent structural reds has only been recently understood, when researchers found that one of the reasons is that structures that reflect low-energy red also reflect high-energy blue (which reflects more intensely), making the final color always look like a shade of purple.

These and many other insights are helping scientists develop new types of structurally colored materials for exciting applications, which include coatings, paints, textiles, cosmetics, camouflage, art, and even a security feature for Australian bank notes and cards. And because the physics of structural colors is rooted in the physics of light, many applications go beyond colors and into sustainability—for example, designing coatings that reflect light selectively to cool down a space. It’s an exciting time for the study of color.

Learn more about Victoria Hwang’s work on designing structural colors inspired by nature.

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