Purple is an enigmatic color that has long fascinated artists, designers, and scientists alike. Unlike the primary colors red, blue, and yellow, which can be used to create all other colors, purple occupies a more complex position in color theory and human perception. So why is purple considered a primary color? The answer lies in the physical properties of light, the biology of human color vision, and the history of purple as a rare and highly prized pigment.
The Physics of Light and Color
To understand why purple is a primary color, we first need to understand some basics about light and color. Visible light is part of the electromagnetic spectrum, and ranges in wavelength from about 400 nanometers (violet) to 700 nm (red). A key principle is that each wavelength corresponds to a distinct color. For example:
| Wavelength (nm) | Color |
|---|---|
| 400-435 | Violet |
| 435-480 | Blue |
| 480-490 | Cyan |
| 490-500 | Green |
| 500-565 | Yellow |
| 565-590 | Orange |
| 590-700 | Red |
When all wavelengths of visible light mix together, we perceive this as white light. However, we see distinct colors when only a limited range of wavelengths reaches our eyes. For example, a lemon appears yellow because it absorbs most other colors and reflects light predominantly in the yellow wavelength range.
So where does purple fit in? There is no single wavelength that corresponds to purple in the visible spectrum. However, our eyes perceive purple when they receive a combination of long and short wavelengths – specifically red (long) and blue/violet (short). This stimulation causes our brain to interpret the color as purple.
Human Color Vision
To fully grasp purple’s status as a primary color, we need to delve a bit into the biology of human color vision. Normal color vision relies on specialized cells in the retina called cones. There are three types of cones that are each sensitive to different wavelength ranges:
| Cone type | Peak sensitivity |
|---|---|
| S cones (short) | 420 nm (blue/violet) |
| M cones (medium) | 534 nm (green) |
| L cones (long) | 564 nm (yellow/red) |
When light hits the cones, signals are sent to the visual cortex of the brain, which interprets the specific combination of stimulations from the three cone types as a particular color. Critically, there are no cone cells devoted solely to detecting purple light. Yet we perceive the color purple thanks to simultaneous stimulation of the S and L cones.
Trichromatic Color Theory
In the 18th century, Tobias Mayer proposed that all colors could be created from just three primary hues. In the 1850s, James Clerk Maxwell demonstrated this experimentally by producing color photographs using only red, green and blue filters. This became known as the trichromatic theory.
The trichromatic theory matches up neatly with our three types of cones. It states that any color can be produced by combining appropriate amounts of three primary colors:
| Primary color | Cone stimulated |
|---|---|
| Red | L cones |
| Green | M cones |
| Blue | S cones |
This forms the basis for many color mixing systems, including computer monitors and televisions which create colors by emitting different amounts of red, green and blue light.
Notably, purple is not one of the primary colors in trichromatic theory. But it can be produced by mixing red and blue primaries, thereby stimulating L and S cones simultaneously. In this way, the trichromatic model accounts for our ability to perceive purple hues.
The Unique Nature of Purple
Unlike other colors, purple does not correspond to any single wavelength of light. Our brains essentially create it as an extra sensory perception when we view a combination of red and blue/violet wavelengths. In this sense, purple is an invented color that exists only in our visual cortex, not in the physical light spectrum itself. This grants purple a unique and almost mystical quality unlike any other hue.
Artists have long recognized purple’s special nature. In painting, purple pigments were historically derived from rare mineral deposits and even snail secretions, making them tremendously expensive. As a result, purple fabric was reserved for royalty and the very wealthy. The rarity and prestige of purple continues even today in symbols like royal robes and the Catholic bishop’s cassock.
So in summary, purple is considered a primary color for the following reasons:
– It stimulates two different cone cell types (S and L) simultaneously.
– According to trichromatic theory, all colors can be mixed from three primaries including red and blue.
– Purple is perceived when red and blue light mix, activating two primary color channels.
– Purple has no single corresponding wavelength on the light spectrum.
– Our brains construct the perception of purple, giving it a unique sensory quality.
For these reasons, purple occupies a distinctive place as a non-spectral, extra-spectral, or invented primary color. Mixing appropriate amounts of red and blue light allows us to produce the full range of purple hues that exist only in our minds. So while purple may not appear in a literal rainbow, it is considered a primary color in human visual perception.
Properties of Purple Light
Although purple does not have its own wavelength, mixing red and blue light together produces light that does have unique properties worth examining:
– Wavelength: Between 400-550nm approximately
– Frequency: Around 700-850 THz
– Energy: 2.75-3.26 eV at peak perception
This light stimulates both S and L cones, but more weakly than pure violet or red light. This is what gives purple its distinctive hue.
Additionally, purple light:
– Travels at the same speed of light (299,792 km/s) as other colors
– Can be reflected, refracted and dispersed into a spectrum
– Is visible to people with normal color vision
– Is perceived differently by those with color blindness
– Is effectively invisible to mantis shrimps, birds, and other tetrachromats
So in terms of its physical behavior, purple light acts like any other composite wavelength of visible light when produced. Only our visual processing gives its unique color significance.
Mixing Pigments vs. Light
An important distinction emerges when we mix colored pigments vs. mixing colored light. With pigments, mixing primary colors produces secondary and tertiary colors through subtraction:
| Primary pigment colors |
|---|
| Yellow |
| Magenta (violet/red) |
| Cyan (blue/green) |
For example, mixing yellow and magenta pigments absorbs green wavelengths, producing red light to our eyes. But with light itself, mixing primaries uses additive color:
| Primary light colors |
|---|
| Red |
| Green |
| Blue |
Here, combining red and blue makes purple through summation, not absorption. This discrepancy can create confusion about primary colors, but both additive (light) and subtractive (pigment) models are valid in different contexts. Purple’s status as a primary concerns mixing light, not paints or dyes.
Purple vs. Violet
In color theory, purple and violet are sometimes used interchangeably, but there is a subtle difference:
| Color | Definition |
|---|---|
| Violet | Extreme end of visible spectrum (380-450nm) |
| Purple | Non-spectral mix of red + blue |
So violet indicates a spectral wavelength near the ultraviolet boundary, while purple refers more broadly to hues between red and blue. All violets are purples, you might say, but not all purples are strictly violets. This distinction is not always made, however – especially in art or fashion where “purple” is commonly used to refer to any hue between magenta and indigo.
Shades of Purple
Within the range of purples, many delightful shades and hues exist. Some common purple varieties include:
| Name | Characteristics |
|---|---|
| Lavender | Light, soft purple with more blue |
| Lilac | Pale purple with subtle pink |
| Mauve | Grayish purple with red tones |
| Wisteria | Bluish-purple named after the flower |
| Orchid | Rich purple with fuchsia tones |
| Amethyst | Vivid purple, named after the gemstone |
| Byzantium | Deep reddish-purple hue |
| Tyrian | Royal purple toned with crimson |
And those are just a few examples. With mixing and tinting, the diversity of purple hues is essentially unlimited. Visit any acrylic paint aisle and you’re sure to find dozens of imaginatively-named purples!
Psychology of Purple
How do different purple hues impact us psychologically? Here’s a quick overview:
| Shade | Psychological Effect |
|---|---|
| Light purples | Soothing, romantic, fanciful |
| Moderate purples | Spiritual, contemplative, mystical |
| Dark purples | Royal, luxurious, dramatic |
In color psychology:
– Lighter purples promote relaxation and romance
– Bright purples encourage creativity and imagination
– Darker purples convey luxury, ambition, power
Purple is also associated with wisdom, dignity, independence, and magic. Sensations of awe or spirituality can emerge from intermediate hues. Overall, purple carries a rare enchantment unlike any other color.
Uses of Purple
Thanks to its unique perceptual properties, purple finds many uses:
| Field | Uses of Purple |
|---|---|
| Fashion | Conveys extravagance and individuality |
| Interiors | Adds intrigue, creativity and regal flair |
| Flora/Fauna | Attracts pollinators to flowers; camouflage |
| Art | Conveys mood, emotion, and symbolism |
| Psychology | Used in therapy for relaxation or introspection |
| Physics | Studied to understand light and color perception |
| Medicine | Phototherapy for jaundice, seasonal affective disorder |
From orchids to amethysts, Eggplant emojis to Prince albums, purple reigns supreme across many aspects of human life and culture.
Conclusion
Purple holds a special place in color theory as a non-spectral primary color created by our visual perception. Mixing lights at the red and violet ends of the spectrum generates the wide gamut of purple hues that cannot be attributed to any single wavelength of light. Yet our brains fill in the sensory experience of purple colors beautifully. Trichromatic theory and the biology of human color vision explain why purple is considered a primary color, despite its unique relationship to the visible spectrum. When we view purple, we are glimpsing an extraordinary interplay between physics, physiology, and psychology that reveals just how clever and capricious our evolved senses can be.
