The color violet is a mix of red and blue light. When red and blue light are combined, the human eye perceives the color violet. This is due to the way our eyes detect color using light-sensitive cells called cones. There are three types of cones that allow us to see the colors red, green, and blue. By mixing different amounts of these three primary colors of light, our eyes can perceive all the colors in the visible spectrum. Understanding how combining red, green, and blue light creates violet provides insight into the physics and biology of human color vision.
How the Eye Sees Color
The human eye contains two main types of light-sensing cells: rods and cones. Rods allow us to see in low light conditions, like at night. Cones are concentrated in the fovea, the central part of the retina, and allow us to see color.
There are three types of cones, each containing a different photopigment that is most sensitive to either red, green or blue light. The absorption spectra of the photopigments in the three cone types overlap, meaning they can each detect a range of wavelengths. But each type is maximally sensitive to a different part of the visible light spectrum:
| Cone type | Peak sensitivity wavelength |
|---|---|
| S-cones (short) | 420 nm (blue) |
| M-cones (medium) | 530 nm (green) |
| L-cones (long) | 560 nm (red) |
When light enters the eye and strikes the cones, the cones send signals to the brain indicating how much red, green or blue light was detected. The brain then interprets these signals as different colors. This is called the trichromatic theory of color vision.
For example, when we look at an object that appears red, it is reflecting more red light and less green and blue light. The L-cones detect a stronger red light signal while the M- and S-cones detect weaker green and blue signals. The brain interprets this combination of cone signals as the color red.
Additive Color Mixing with RGB
RGB stands for the three primary colors of light: red, green, and blue. Televisions, computer monitors, phones and other digital displays create colors by mixing amounts of red, green and blue light. This is known as additive color mixing, because the light from each RGB component adds together to create other colors.
With additive RGB color mixing, combining red and green light creates yellow, red and blue makes magenta, and blue and green produces cyan. When red, green and blue light are combined at full intensity, the eye perceives the mix as white light. By varying the brightness of the individual RGB components, any color in the visible light spectrum can be reproduced.
This additive color process is different than mixing paints, inks or dyes, which use a subtractive color model based on cyan, magenta and yellow pigments. With paints and inks, individual pigments subtract certain wavelengths of light and selectively reflect others, creating different colors for our eyes to see.
How Violet Arises from Mixing Red and Blue Light
As described above, violet light stimulates the S-cones in our eyes more strongly than the M- and L-cones. Violet has a wavelength of approximately 380-450 nm, at the short wavelength end of the visible spectrum. It sits between blue and ultraviolet.
When red light and blue light mix together in equal amounts, the combination excites the S-cones and L-cones almost equally. The M-cones receive less stimulation. This cone activation pattern is interpreted by the brain as the color violet.
Specifically, red light strongly stimulates the L-cones due to its long wavelength (~700 nm). Blue light, with its shorter wavelength peak around 420-450 nm, stimulates the S-cones more strongly. Combining these two lights provides excitation of both the L and S cones, without as much M cone stimulation. The brain perceives this as violet.
By comparison, mixing red and green light activates L and M cones, which is seen as yellow. Mixing blue and green stimulates M and S cones, generating cyan.
Properties of Violet Light
The exact wavelength range for violet light is between 380-450 nm. This sits at the short wavelength end of the visible spectrum, next to blue light and ultraviolet. Some key properties of violet light include:
- Wavelength range: 380-450 nm
- Frequency range: 668-790 THz
- Photon energy range: 2.75-3.26 eV
- Violet has the shortest wavelength of all visible colors
- Sits between blue (450-495 nm) and ultraviolet (
- Strongly stimulates the S-cones in our eyes
In the color spectrum, violet lies between blue and the invisible ultraviolet rays just beyond the visible edge. Violet light is at the high frequency, short wavelength end of the optical spectrum. It contains photons with energies of 2.75-3.26 electron volts.
These high-energy violet photons can cause chemical reactions, which is why ultraviolet light from the Sun allows our skin to produce vitamin D but also causes sunburns.
Violet Light in Nature
In nature, violet light is present in rainbows and can be seen when sunlight passes through a prism. As white light disperses into the color spectrum, violet appears at the short wavelength end after blue.
The violet color in rainbows is faint and often hard to discern due to its low wavelength sensitivity in human eyes. The sky appears blue (rather than violet) because the atmosphere scatters blue light more strongly than violet.
However, many birds, insects and flowers have sensitivity to ultraviolet and violet wavelengths that humans lack. Butterflies like the cabbage white and brimstone can see ultraviolet patterns on flowers that guide them to nectar. Bees also see in ultraviolet, helping them find their way to pollen.
Violet light from the Sun breaks down ozone high in the atmosphere. This ozone layer protects life by absorbing harmful ultraviolet radiation.
Violet Dyes, Pigments and Paints
While violet can be produced with light, creating vibrant violet colors in paints, dyes and pigments is more challenging. Violet sits at the edge of the visible spectrum, making it difficult for pigments to selectively reflect violet wavelengths.
In 1685, the first synthetic purple dye was created from coal tar. Other early synthetic dyes like mauveine and Perkin’s mauve also showed violet hues. Today, organic chemistry can produce vivid violet pigments for paints, inks, textiles and cosmetics.
Common violet pigments today include:
- D&C Violet No. 2 – a triarylmethane dye used in cosmetics
- Manganese violet – an inorganic pigment made from manganese phosphate
- Perylene violet – an organic synthesized perylene pigment
- Quinacridone violet – a synthetic organic quinacridone dye
The absorption and reflection of violet dyes involves quantum mechanics. The photons that excite electrons in the pigment molecules into higher energy states have energies matched to violet wavelengths. Engineers carefully design violet dye molecules to absorb and reflect the desired violet colors.
Violet Light and Human Vision
As described previously, violet light strongly stimulates the S-cones in our eyes, which are sensitive to blue light. However, the S-cones are relatively sparse compared to red and green sensitive cones. Moreover, the visual system processes signals from S-cones differently.
This results in violet colors appearing less bright to humans compared to other colors. While violet has a high luminous efficiency in terms of triggering S-cones, the brain’s interpretation of S-cone signals gives violet a low luminous intensity.
Violet also sits at the edge of human vision, next to invisible ultraviolet light. For these reasons, violet is considered a spectral color with low brightness or luminance. Artist paint mixes violet with white to strengthen its intensity on canvases and compensate for its low luminance.
Applications of Violet Light
Some uses and applications of violet light include:
- Phototherapy – Violet light is used in light therapy and phototherapy equipment to treat skin conditions and mood disorders.
- Black lights – Violet radiation from black lights causes certain dyes and pigments to fluoresce, creating neon colors under the UV illumination.
- Lasers – Violet diode lasers are used for laser projection and laser TV displays because the short wavelength produces a smaller beam focus.
- Optical storage – CDs, DVDs and Blu-ray Discs use violet lasers (405 nm) for reading and writing data, allowing for greater storage densities.
- UV curing – Violet wavelengths enable fast UV curing of inks, coatings and adhesives that contain photosensitive chemicals.
Because violet sits next to ultraviolet wavelengths just outside the visible edge, it can excite fluorescence and chemical reactions like UV light. At the same time, most violet light is within the visible spectrum, allowing us to see the vivid colors associated with violet laser displays, dyes, and pigments.
Conclusion
In summary, violet light arises in the visible color spectrum from combining red and blue light. Red light stimulates the L-cones in our eyes while blue light excites the S-cones. When red and blue mix, the combination of L and S cone stimulation is perceived as violet. This additive color mixing process allows RGB displays and projectors to create the color violet. While dimmer than other colors due to our eye’s lower sensitivity to S-cone signals, violet light has unique properties including short wavelengths and high frequencies that enable many industrial and scientific applications. Understanding how combining the primary colors of red, green and blue light generates the vivid palette of the visible spectrum provides insight into the physics, biology, and perception of color vision.
