Red is a primary color that exists on the visible spectrum of light. It sits between orange and violet and has a wavelength of approximately 620-740 nanometers. Red is an additive primary color, meaning that when red light is combined with green and blue light, all other colors can be created. But where does the perception of “redness” actually come from?
The Science of Color Vision
To understand where the color red comes from, we first need to understand a bit about how human color vision works. Color is not an inherent property of light or objects – it is created by our visual system. Light hits the photoreceptor cells in our eyes, which contain pigments that are sensitive to different wavelengths of light. There are three types of cones cells:
- S cones – most sensitive to short wavelengths (blue light)
- M cones – most sensitive to medium wavelengths (green light)
- L cones – most sensitive to long wavelengths (red light)
When light hits the retina, it stimulates the cone cells differently depending on its wavelength. The relative stimulation of the three cone types is interpreted by the visual cortex of our brain as different hues. Red light strongly stimulates the L cones but only weakly stimulates the S and M cones. This cone stimulation pattern is what creates the perception of “redness”.
Properties of Red Light
As mentioned above, red sits on the long wavelength end of the visible spectrum, between roughly 620-740 nm. It has a lower frequency and energy than other visible colors. Here are some of the key properties of red light:
- Wavelength range: ~620-740 nm
- Frequency range: ~405-480 THz
- Photon energy range: ~1.77-2.00 eV
Red has the longest wavelength and lowest frequency/energy of all the rainbow colors. Violet and blue light have shorter wavelengths and higher frequencies and energy. Red’s long wavelength means it tends to bend the least when passing through a prism, compared to other colors.
Where Red Fits in the Color Spectrum
Here is a table showing where red fits within the visible color spectrum:
| Color | Wavelength range (nm) |
|---|---|
| Violet | 380-450 |
| Blue | 450-495 |
| Green | 495-570 |
| Yellow | 570-590 |
| Orange | 590-620 |
| Red | 620-740 |
As you can see, red occupies the long wavelength end of the visible spectrum, while violet occupies the short wavelength end. The varying wavelengths of light from this spectrum allow us to perceive all the colors of the rainbow.
Where We See Red in Nature
Red appears commonly in nature, both in plants and animals. Here are some common natural occurrences of red coloration:
- Cardinal birds have bright red plumage
- Red roses and poinsettias have red petals
- Strawberries and cherries are red when ripe
- Red sunsets occur when sunlight passes through more atmosphere
- Blood is red due to the hemoglobin protein
- Autumn leaves turn red as green chlorophyll breaks down
The red color usually comes from pigments in the surface of objects that strongly absorb other wavelengths, reflecting mainly red to our eyes. For example, the red anthocyanin pigments in strawberries absorb green and blue light, leaving mainly red to reflect back.
How Red Pigments Get Their Color
Red pigments like those found in plants, animals, paints, dyes, and minerals get their rich color from their chemical composition, which allows them to absorb certain wavelengths of light more than others. Here are some common red pigments and how their structures impart color:
- Anthocyanins – Water-soluble plant pigments that appear red due to resonance stabilization of their conjugated chromophore.
- Lycopine – The red carotenoid pigment found in tomatoes.
- Hemoglobin – Contains a heme group with a conjugated chain that strongly absorbs green light.
- Cadmium red – Inorganic pigment made with cadmium that absorbs blue/green light.
- Vermillion – Mercury sulfide compound that reflects mainly red when finely ground.
In all these cases, red color comes from the selective absorption of specific wavelengths of light by the pigment’s molecular or crystal structure. This leaves mainly red light to be reflected back to our eyes.
How Red Dyes and Paints Work
Red dyes and paints work by transmitting or reflecting mainly red wavelengths while absorbing most other visible wavelengths. Here are some common red dyes and how they work:
- Allura Red AC – Synthetic red azo dye used in foods. Absorbs green/blue light.
- Rhodamine B – Red fluorescent dye used as a tracer dye and stain. Has a conjugated structure that transmits red.
- Congo Red – Sodium salt of benzidinediazo-bis-1-naphthylamine-4-sulfonic acid. Strongly absorbs green and blue light.
And some common red paint pigments:
- Cadmium red – Inorganic pigment that strongly absorbs blue/green light.
- Naphthol red – Organic pigment with a red diazo dye. Absorbs blue/green.
- Ferric oxide – Iron oxide-hydroxide mineral pigment that reflects mainly red and infrared light.
By selectively transmitting or reflecting red wavelengths, these dyes and pigments appear intensely red to our eyes.
How Red Light Affects Us
Red light has several interesting effects on human health, mood, behavior, and more. Here are some ways red light can affect people:
- Increases heart rate and blood pressure due to stimulation of the sympathetic nervous system.
- Enhances vigilance, alertness, and arousal.
- Linked to increased appetite and impulsivity.
- Used in light therapy to treat seasonal affective disorder and skin conditions.
- Makes food look more appealing and sweet.
So while red has the lowest light energy of the rainbow, it has powerful effects on the human body and mind. Exposure to red light should therefore be carefully controlled depending on the context.
Importance of Red in Human Culture
The color red holds much symbolic meaning in human cultures all around the world. Here are some of the significance and common associations of red:
- Danger, caution, stop
- Love, passion, romance
- Heat, fire, anger
- Excitement, intensity, speed
- Blood, war, violence
- Luck and celebration (in some Asian cultures)
Red evokes strong emotions and symbolism for people depending on the context. It is commonly used in flags, signs, clothes, and advertisements to convey different meanings.
Conclusion
In summary, red is a primary color with the longest visible wavelength that strongly stimulates our L cone photoreceptor cells. It occupies the long wavelength end of the visible spectrum. Red commonly appears in nature due to pigments that absorb other wavelengths, leaving mainly red to reflect back. Red dyes, paints, and light can have significant impacts on humans. And red holds much cultural symbolism related to passion, danger, and more. So next time you see something red, consider all the fascinating science and perception that determines its beautiful color.