Red is a color that is associated with passion, energy, and warmth. But what makes things actually appear red to our eyes? The answer lies in the properties of light and the way our eyes perceive different wavelengths.
When white light shines on an object, some wavelengths are absorbed while others are reflected. The reflected wavelengths determine what color our eyes see. Red objects reflect longer wavelengths of light – around 700 nanometers – while absorbing other wavelengths.
Our eyes have special receptor cells called cones that are sensitive to different wavelengths of light. When the cones that detect red light are stimulated, our brain interprets this as the color red. So any object that reflects a lot of 700nm light back to our eyes will appear red.
Why do different materials look red?
There are several ways that materials can reflect red light and take on a red color:
| Material | Reason for red color |
|---|---|
| Ruby | Trace amounts of chromium absorb blue and yellow light, leaving red |
| Blood | Hemoglobin protein strongly reflects red light when oxygenated |
| Red paint | Contains red pigments that absorb other colors and reflect red |
| Red flowers | Get red pigments from plant molecules like anthocyanin |
| Red clothes | Dyed with synthetic red dye molecules |
As you can see, the red color can originate from a variety of molecules and compounds that happen to absorb non-red wavelengths and reflect red ones.
Transition metals like chromium, organic pigments from plants, and synthetic dyes are all examples of chemicals that selectively reflect and transmit red light. When these red-reflecting molecules are present in a material, it will appear some shade of red.
Primary colors of light vs. pigment
When talking about color, it’s important to distinguish between the primary colors of light and the primary colors of pigment.
The primary colors of light are red, green, and blue. Mixing different amounts of these colored lights can produce most other colors. This is the principle used in TVs, computer monitors, and other displays to create color.
The primary colors of pigment – like paint and dyes – are cyan, magenta and yellow. This is because pigments work by absorbing some wavelengths and reflecting others. For example, a red pigment absorbs green and blue light while reflecting red. Combining primary pigments in different ratios can also produce many colors.
So red light is a primary color, while a red pigment is actually reflecting just red and absorbing other colors. This distinction explains why different materials can appear red even though they contain different pigments.
Red in nature
Red coloration is very common in the plant and animal worlds. Evolution has led to many instances where red coloration developed because it provided some advantage.
One reason is that the red pigment anthocyanin offers protection against sun damage. Many red fruits, leaves, and flowers contain anthocyanin. The red color warns predators that the plant may be poisonous or unpalatable.
Animals can also benefit from red coloration. Some use it as a warning, like the red-bellied black snake. Others have red markings that serve as camouflage in their natural environment, like red grouse in the heather.
Humans likely associate red with danger and passion because of our instinctive reactions to red in nature. Red has signaled threats throughout our evolution, but also ripeness and sweet, nutrient-rich foods like berries. This may drive red’s emotional effect.
How eyes see color
To understand why certain materials appear red, it’s helpful to understand a bit about how eyes detect color.
Human eyes have two types of light receptors – rods and cones. The rods detect brightness and motion, while the cones are responsible for detecting color.
| Cone type | Color detected |
|---|---|
| S cones (short wavelength) | Blue light |
| M cones (medium wavelength) | Green light |
| L cones (long wavelength) | Red light |
We have three types of cones that are activated by different wavelengths of light. When the brain receives signals from the cones, it interprets them as color.
For example, when only the L cones (red cones) are activated, we see red. If the L and M cones (red and green) are activated, we see yellow. Our brain puts together the combinations of cone responses to let us perceive the whole spectrum of colors.
So in summary, red objects reflect light that activates the L cones maximally. This cone stimulation is interpreted as red by the visual processing in our brains.
How red pigments work
Pigments appear red because they absorb some wavelengths of light while reflecting red wavelengths. This selective absorption and reflection is what gives pigments their characteristic color.
The molecular structure of pigments determines which wavelengths get absorbed. Factors can include:
- Conjugated double bonds – Alternating single and double bonds can absorb specific wavelengths
- Aromatic rings – Found in molecules like anthocyanin, absorb non-red colors
- Transition metals – Metals like chromium absorb yellow and blue light
- Attached functional groups – Can shift the absorbed wavelengths
Engineering pigments to have specific absorption spectra allows chemists to produce a huge variety of colored compounds. Most synthetic red dyes and pigments are made by tweaking the molecular structure of organic chemicals to fine-tune their light absorption.
In general, adjusting the types of bonds and rings will control what colors a pigment molecule absorbs when hit with white light. This absorption profile directly affects what colored light gets reflected and seen by our eye’s red cones.
How red is produced technologically
While nature has evolved many red organic pigments, humans have also engineered purely synthetic red dyes and pigments. These allow us to color a wide range of commercial products red.
Some examples of synthetic red dyes and pigments include:
- Azo dyes – Contain nitrogen double bonds, used in clothing
- Quinacridone – Organic compound used in plastics and ink
- Perylene red – Made from hydrocarbons, used in automotive paints
- Naphthol red – Derived from napthalene compounds, used in artist paints
- Phthalocyanine – Contains rings of carbon and nitrogen, used for pigments
Chemists can modify these structures to produce specific shades and properties. By targeting how the molecules absorb and reflect light, red colorants can be designed on the molecular level.
These synthetic red dyes allow us to color fabrics, paints, plastics and many other commercial products red. This expands the palette of reds beyond what is available naturally.
Why do different reds have different shades?
There are many shades of red – from blood red to fire engine red to rust red. The specific hue depends on the material’s exact absorption spectrum.
While all red materials absorb green and blue light, they can absorb differing amounts of orange and yellow wavelengths.
For example:
- Yellowish reds absorb more blue light
- Bluish reds absorb more yellow and orange
- Bright red absorbs greens and blues evenly
- Dark red absorbs across the whole visible spectrum
The balance of wavelengths getting absorbed versus reflected back determines the red’s final shade. Minor variations in a red pigment’s molecular structure can shift which colors it takes in.
Environmental factors can also change a red’s appearance. Other colors surrounding it can shift the way the eye perceives a red object. The quality of ambient light also impacts the shades we observe.
Conclusion
Red is a vivid, emotionally intense color that nature has evolved to signal many meanings. Materials appear red because their molecular structure absorbs non-red wavelengths, only reflecting light detected by our eye’s red cones. Many natural and synthetic pigments selectively filter light in this way. The specific atomic makeup of pigments determines the nuances of the final red color we observe. So the next time you see something red, you’ll know it owes its appearance to the fascinating interactions between light, chemistry, and human perception.
