The chemistry of colors
Article by: Camilla Fezzi, on 08 September 2023, at 03:17 pm PDT
Color is one of life's greatest wonders. It bursts forth from flowers to attract pollinators and fills our natural world with vibrancy. But the origins of color run much deeper than just decoration or attraction. At the most fundamental level, color exists because of the interaction between light and matter. The story of color unveils nature's exquisite design at the microscopic level.
All objects emit or reflect light of various wavelengths across the visible light spectrum. White light, which we perceive when all wavelengths combine, contains wavelengths of every color. When white light strikes an object, some wavelengths are absorbed while others are reflected or transmitted. It is these reflected wavelengths that reach our eyes and which our brains then interpret as color.
Absorption underlies color at the atomic scale. Every molecule is uniquely structured, containing certain arrangements of atoms. The distances between atoms and their electrochemical bonds determine which wavelengths of light a molecule can readily accept and which it reflects. Common organic dyes work this way, absorbing certain colors while reflecting complimentary ones. Chlorophyll, the molecule driving photosynthesis, strongly absorbs wavelengths in the violet-blue and orange-red ranges, reflecting the green light that gives leaves their appearance.
Beyond molecules, color arises from microscopic surface properties. Materials like glossy leaves and iridescent jewels contain arrays of nanostructures that manipulate light in specialized ways. The spacing of these nanoscale ridges, pits or spheres determines which colors constructively interfere and bounce back to our eyes. Examples include butterfly wings with colors derived from diffraction gratings just nanometers thick.
Moving to larger scales, textures and structures also induce color. Think of the sparkling rainbow sheen of an oil slick or mother-of-pearl. Their brilliant hues stem from microscopic pillars or spheres which scatter wavelengths of light at specific angles. Similarly, peacock tail feathers derive vibrant displays from the spacing of keratin rods within each individual filament. Judging by these feats of nano-engineering, nature has long since mastered the science of structural color.
The most reflective objects, such as mirrors and the scales of some exotic beetles, bounce nearly all wavelengths. We see them as silver or white. While absorbing surfaces, like charcoal or asphalt, soak up visible light, appearing black or dark gray. Most materials fall in between, preferentially interacting with select frequencies to project colors from across the rainbow.
Beyond aesthetics, this interplay between light and physical structures underlies vision itself. Humankind evolved three types of color receptors in the eye, each most sensitive to a distinct part of the visible spectrum. Through expression of pigments called photoreceptors, our retinas absorb cues needed for distinguishing wavelengths. We then perceive blue, green or red light intensity differences which the brain combines into the rich tapestry of colors enhancing daily life.
Given its origin in fundamental physical principles, color is perhaps unsurprising in nature. Yet this interaction between light, molecules and nanoscale architects never ceases to instill wonder. From majestic sunsets to the iridescent wing of a butterfly, the visible worlds created through absorption and scattering showcase nature's infinite creativity in painting our earth with vibrant colors. Truly, color energizes both our lives and spirits through both its embedded beauty and its grounding in the harmonious laws governing our universe. Its genesis reminds us to marvel not only at the magnificence of our multicolored reality but also at its deep roots in elegant, underlying science.
As we've seen, color arises through the interaction of light with matter at microscopic scales. But what determines a material's ability to absorb and reflect various wavelengths? The answer lies in atomic and molecular chemistry.
All atoms contain electrons that can move between different energy levels. When electrons absorb the energy of a photon, they jump to a higher orbital. The exact distance they jump, known as the "energy gap", corresponds to a specific wavelength of visible light. Only photons with matching energies can boost electrons upwards.
Different elements and compounds have unique electronic structures defined by their chemical makeup. This dictates which wavelengths atoms and molecules preferentially accept or dismiss. For example, the electron configuration of chlorophyll pigment molecules only permits photons in the violet-blue and orange-red regions to induce necessary jumps. All other frequencies pass through, leaving the molecule's reflected hue as green.
The versatility of molecular color properties stems from variations in bonding. Organic dyes often contain aromatic ring structures that localize electrons into clouds easily interacting with light. Sulfur, nitrogen or oxygen atoms within these rings enable new energy transitions by donating or accepting electrons into hybridized orbitals. Even minor changes in functional groups or double bonds can substantially shift which wavelengths are absorbed.
Conjugated systems, where alternating single and double bonds allow electron delocalization, give especially vibrant colors. Common examples include yellowcurcumin in tumeric and red lycopene found in tomatoes. Carotenoids, like those putting an orange sheen on carrots, all contain long stretches of conjugation. This lowers the energy required for electrons to migrate up and down the molecular "wire", selectively soaking up wavelengths.
Chemistry can further tune color through substituent effects. Electron-donating groups like -OH or -NR2 attached to chromophores push the pi-cloud's energy down, shifting absorption to longer wavelengths and imparting a red-purple hue. Electron-withdrawing substituents like -COOH or -NO2 have the opposite impact, shortening wavelengths absorbed and producing blues or greens. Nature plays with these substituents to generate a wide assortment of molecular reds, oranges and yellows essential for processes like photosynthesis and vision.
In addition to pigments, scattering participates in many structural color phenomena. Materials like feathers, beetle carapaces and butterfly wings contain one-dimensional arrays of rods, spheres or other microscopic elements with periodic spacing on the order of visible wavelengths. Engineers use concepts like Bragg's law to calculate the wavelength selectively reflected back or transmitted based on the inter-reflector distance - allowing whole new color palettes through architecture alone.
From the nanoscale up, nature wields chemistry's building blocks with artistic grace to not only perform vital functions, but also produce the lush visible world surrounding us. An deeper understanding of molecular interactions with light continues revealing the exquisite science integrated into even something as seemingly simple as a rainbow.