Purple is indeed considered a color, but its status as a spectral color is somewhat ambiguous compared to other colors like red, blue, and green. To understand why, we first need to look at what defines a color as “spectral” versus “non-spectral.”
What is a spectral color?
The visible spectrum of light that humans can see ranges from red at the long wavelength end through orange, yellow, green, blue, indigo, and violet at the short wavelength end. Spectral colors are those wavelengths of light within the visible spectrum that correspond to specific colors as detected by our eyes.
Red, orange, yellow, green, blue, and violet wavelengths exist as pure spectral colors within the visible light spectrum. These colors can be reproduced with a single wavelength of light at that specific frequency. For example:
| Color | Wavelength range |
|---|---|
| Red | ~700-635 nm |
| Orange | ~635-590 nm |
| Yellow | ~590-570 nm |
| Green | ~570-500 nm |
| Blue | ~500-450 nm |
| Violet | ~450-400 nm |
When a single wavelength of light within one of these ranges enters our eye, our visual system perceives it as the corresponding spectral color. So red, orange, yellow, green, blue, and violet wavelengths are all considered pure spectral colors.
Is purple a spectral color?
Unlike the other colors mentioned, there is no single wavelength of light that corresponds to the color purple. Purple is not present as a pure color within the visible light spectrum. There is no “purple wavelength.”
Instead, purple arises when our eyes detect red and blue light simultaneously. The mixing of these wavelengths creates the perception of purple. So purple is considered a non-spectral color, meaning it exists as a mixture of two spectral colors rather than a pure color itself.
The mixing of red and blue light
Within the visible spectrum, the longest wavelengths we perceive as red and the shortest wavelengths we perceive as blue are at opposite ends of the spectrum. Red light has wavelengths around 700-635 nm, while blue light is around 500-450 nm.
When red and blue wavelengths enter our eye at the same time, the red cones and blue cones in our retinas both get stimulated. Our visual system interprets this combined stimulation as the non-spectral color purple.
This additive mixing of red and blue light to produce purple happens due to the way our eyes detect color. The stimulation of both the red and blue photoreceptor cones essentially “tricks” the brain into seeing the new color purple.
How purple is produced
There are a couple ways that simultaneous red and blue wavelengths can enter our eyes to produce the perception of purple:
- Mixing red and blue colored light directly. This can be done with red and blue stage lights or pigments that reflect red and blue light.
- Viewing a violet wavelength (~400-450 nm) and a red wavelength together. Violet stimulates the blue cones somewhat, while the red wavelength stimulates the red cones.
- Viewing a purplish wavelength near the shortwave end of the visible spectrum around 470-420 nm. This short wavelength stimulates blue cones more strongly than red cones.
In all these cases, both the red and blue photoreceptor cones in the retina get stimulated to some degree. The brain combines the dual stimulation and gives rise to the perception of purple.
Not on the EM spectrum
The visible color spectrum is part of the full electromagnetic (EM) spectrum. This entire EM spectrum represents all the possible wavelengths or frequencies of electromagnetic radiation. Within this full spectrum, visible light makes up only a small slice.
Importantly, purple does not occupy any space within the EM spectrum. As a non-spectral color arising from combined wavelengths, there is no specific “purple wavelength” at some point along the EM spectrum. It exists only as a visual perception, not as a pure wavelength of EM radiation.
A distinct color experience
While not present as a pure single wavelength, purple is still perceived as its own distinct color by the human visual system. When you look at purple, you have a specific color experience different from red, blue, or any other color.
Our trichromatic vision system allows the mixing and perception of non-spectral colors like purple and magenta. So even though purple light doesn’t exist by itself as a pure wavelength, we are still able to see and experience it as its own unique color.
Comparable to magenta
Purple’s status as a non-spectral color made from combining wavelengths of light is actually very similar to that of magenta. Magenta is also not found as a singular wavelength within the visible spectrum.
Instead, magenta arises when our eyes detect red and violet light simultaneously. When the red cones and blue cones are stimulated by these wavelengths, our brains interpret the combination as magenta.
So both purple and magenta exist as non-spectral colors that we can perceive through the mixing of two different wavelengths of spectral light.
Not everyone sees it the same way
While most people perceive purple when both red and blue light enters the eye, this is not universally true. A rare condition called blue cone monochromacy means some people lack red cones entirely. For these individuals with impaired color vision, combining wavelengths does not produce purple at all – only mixtures of blue and gray are seen.
So the experience of purple when red and blue mix relies on having both red and blue photoreceptor cones. People missing either cone type may not perceive purple the same way others do.
Shades of purple
Just like other colors, purple can take on many different shades and hues. These depend on the exact wavelengths of red and blue light mixed together, and their relative intensities. Shorter, more intense blue wavelengths mixed with less intense red light will produce a purple with more of a blueish tint. Lower intensity blue mixed with strong red wavelengths results in a purple more closely bordering on magenta.
This variance in the purples we see results from small differences in the stimulation levels of the red and blue photoreceptor cones in our retinas when viewing purple light. Our brains do some complex processing to turn these cone signals into the wide array of purple shades we can perceive.
A popular color
Despite its ambiguous status as a non-spectral color, various shades of purple remain extremely popular. Lavender, lilac, violet, and plum hues of purple are used frequently in art, fashion, marketing, decorating, and more. The color is often associated with royalty, luxury, spirituality, and creativity.
While opinions on favorite colors vary by culture and individual, surveys of western populations often find that purple is one of the most beloved colors. Its rarity in nature along with its ties to wealth and inventiveness help give purple broad appeal.
Uses and applications
Beyond just being aesthetically pleasing, the color purple finds many practical uses thanks to its unique physical properties. A few examples include:
- Imaging and optics – Purple filters are used to enhance contrast in optical instruments like microscopes.
- Jewelry – Purple gemstones like amethyst, tanzanite, and sapphire are prized for their rarity and brilliance.
- Dyes and pigments – Tyrian purple, a dye made from sea snails, was used for royal roman robes.
- Laser technology – Violet and purple lasers are capable of high resolution used in data storage.
- Astronomy – Purple auroras occur due to a mix of red and blue-green light emissions.
From the vivid purple of Prince’s rock band to the soft pastels of Easter eggs, the color purple permeates our lives. While not a pure spectral wavelength, humans have embraced it as a unique and versatile color.
Conclusion
So in summary, is purple a real color? The answer is yes – but it occupies a special place compared to other colors we perceive. Purple is not present as a singular wavelength within the visible light spectrum. Rather, it only arises through the combined stimulation of red and blue photoreceptor cones in the eye when viewing certain mixtures of red and blue wavelengths. This makes purple a non-spectral color. However, our trichromatic vision allows us to perceive purple as its own distinct hue despite its ambiguous status within the spectrum. When it comes to human color experience, purple is just as real and valid as any of the spectral colors.
