Color is a fascinating phenomenon that we experience every day. But what exactly is color, and what causes it to change? In this article, we’ll explore the science behind color changes to gain a deeper understanding of this fundamental aspect of our visual world.
What is color?
To understand what makes colors change, we first need to understand what color is. Color is the way our visual system interprets different wavelengths of light. When light hits an object, some wavelengths are absorbed while others are reflected. The wavelengths that are reflected determine what color we perceive the object to be.
For example, a banana appears yellow because it absorbs most wavelengths of light and reflects back wavelengths around 570 nm, which our eyes and brain interpret as yellow. Grass appears green because it absorbs all wavelengths except those around 510 nm, which we see as green.
The visible spectrum of light that humans can see ranges from about 380 nm (violet) to 740 nm (red). All the colors we experience are mixes of wavelengths within this range.
What makes an object colored?
The color we see an object as depends on two key factors:
- The object’s chemical composition – What wavelengths it absorbs vs reflects
- The color of the light illuminating the object – What wavelengths are available to be reflected
Pigments are chemicals within an object that selectively absorb some wavelengths and reflect others. Different pigments have different absorption spectra, accounting for the diverse colors we see in nature.
But the illumination source also matters. An object may appear a different color under sunlight vs indoor lighting because these light sources contain different mixtures of wavelengths.
How do pigments produce color?
To understand pigments, we need to delve into some chemistry and physics:
- Pigment molecules contain conjugated systems of double bonds that act as chromophores – parts of the molecule that absorb light.
- Depending on their shape and size, different chromophores absorb different wavelengths of visible light.
- When a wavelength is absorbed, its energy causes electrons within the pigment molecule to jump to higher energy levels.
- The reflected color we see is from the wavelengths not absorbed by the pigment.
For example, chlorophyll, the green pigment in most plants, contains a network of conjugated double bonds that strongly absorbs red and blue light. The green wavelengths pass through and are reflected back to our eyes.
What causes color changes in nature?
Now that we understand the basics of color, what leads colors to change in the natural world? There are several common mechanisms:
1. Chemical changes in pigments
One way color changes is through chemical changes to the pigments within an object:
- Plants, animals, and microbes synthesize all sorts of pigments with unique colors.
- As an organism ages, its pigments may oxidize, degrade, or get converted into new molecules with different colors.
- For example, red anthocyanin pigments in leaves degrade to reveal yellow carotenoids during autumn.
2. Physical mixing of pigments
Colors can also change when pigments mix and blend together physically:
- Mixing paints is the classic example – blue and yellow make green.
- Plants mix chlorophylls, carotenoids, anthocyanins, and more to produce their vibrant colors.
- Animals blend pigments together across their skins, feathers, scales, and shells.
3. Structural color effects
Some color changes arise not from pigments, but from microscopic structures that interact with light waves:
- Thin film interference – alternating layers reflect specific wavelengths based on thickness.
- Diffraction gratings – tiny grooves diffract light colors like a prism.
- Localized surface plasmons – metal nanoparticles resonate colors.
These nanoscale architectures produce iridescent structural colors seen in peacock feathers, opals, butterfly wings, and more.
4. Conformational color changes
Shape-shifting molecules can dramatically alter color by reversibly changing conformation:
- Squid proteins change color using conformation switching of pigments.
- Flower pigments change shape in response to environment.
- Chameleons tune color through deposition of nanocrystals regulated by conformation.
What causes color changes in synthetic materials?
Chemists have devised numerous ways to manipulate color for applications like dyes, paints, inks and electronic displays:
1. Oxidation and reduction
Oxidation/reduction (redox) reactions alter the conjugated system of pigment molecules, changing their light absorption:
- Rusting iron turns red as iron oxidizes
- Redox indicators like litmus change color in response to pH
- Photochromic dyes revert color upon light exposure
2. Acid-base reactions
Adding or removing protons also alters pigment absorption:
- Anthocyanin plant pigments change from red in acid to blue in base
- Cabbage juice indicators turn various colors in acid/base
- pH-sensitive smart materials track acidity changes
3. Phase transitions
Physical changes in solid/liquid phase change the dispersion of pigment molecules and light scattering:
- Thermochromic liquid crystals shift color with temperature
- Photochromic glasses darken upon exposure to light
- Gasochromic materials change color in response to gaseous analytes
4. Geometric isomerism
Cis-trans isomerization leads to different geometric configurations with distinct absorption spectra:
- Retinal’s cis-trans switch enables vertebrate vision
- Photoisomerizable pigments interconvert geometric isomers
Applications of color-changing materials
Controlling color change leads to many useful applications:
Color-changing smart materials can detect chemicals, light, temperature, pressure, and other stimuli. Examples include:
- Toxic gas detectors
- pH test strips
- Temperature-sensing liquid crystals
- Pressure-sensitive sheets
Obvious color changes provide clear visual indicators for reactions, transformations, and safety:
- Redox indicators signal electrochemical reactions
- Thermochromic inks monitor freshness and safety
- Photochromic sunglasses darken in sunlight
tuning color change enables vivid, reusable displays:
- Electronic paper (e-paper)
- Reversible thermochromic displays
- Photochromic paints and coatings
Color photography and imaging
Light-sensitive color change enables image capture, storage, and reproduction:
- Color films and photographic papers
- Digital camera LCD and OLED displays
- Cyan, magenta, and yellow inkjet inks
From the complex pigments of animals, plants, and microbes to high-tech display materials, color-changing substances enable life and technology as we know it. As scientists continue discovering new color-tuning mechanisms, we will unlock even more amazing applications that enhance medicine, environmental sensing, consumer products, and quality of life.
Understanding the molecular basis for color and color change is key to designing these next-generation smart materials. With mastery over color change, we can build revolutionary devices limited only by our imagination.