When red and green light are combined, the resulting light appears yellow to human eyes. This is due to the way our eyes perceive color through specialized photoreceptor cells called cones. There are three types of cones that are sensitive to different wavelengths of light – short (blue), medium (green), and long (red).
How the Eye Perceives Color
Inside the retina, the light-sensitive inner surface of the eye, cones are responsible for color vision. Each type of cone is maximally sensitive to a particular wavelength range of visible light. Red cones are most sensitive to long wavelengths, green cones to medium wavelengths, and blue cones to short wavelengths. The brain compares the relative outputs of the different cones to interpret color.
When red light (with a long wavelength of ~700 nm) and green light (with a medium wavelength of ~550 nm) enter the eye together, they stimulate both the red and green cones. The brain compares these signals and perceives the combination as yellow. This is an additive color process, meaning the combination of light stimuli produces a new color sensation.
Cone Response to Red and Green Light
The peak sensitivities of the red, green, and blue cones roughly correspond to particular wavelengths of light. Here is a table showing the peak wavelength and color sensitivity for each cone type:
| Cone type | Peak wavelength (nm) | Color sensitivity |
|---|---|---|
| Red cones | ~700 | Long wavelengths (red) |
| Green cones | ~550 | Medium wavelengths (green) |
| Blue cones | ~450 | Short wavelengths (blue) |
When red light of ~700nm wavelength enters the eye, it strongly stimulates the red cones but less so the green and blue cones. Green light at ~550nm strongly stimulates the green cones. When red and green light mix, both the red and green cones are stimulated fairly evenly, and the brain interprets this combination as yellow.
Additive Color Mixing
Combining wavelengths of colored light to produce new color sensations is known as additive color mixing. This is different from mixing paints, dyes, or inks (subtractive color mixing), where combining pigments produces new colors by absorbing and reflecting different wavelengths selectively.
With lights, the wavelengths simply add together to stimulate multiple cone types simultaneously. Red light stimulates the red cones predominantly, green light the green cones, and a mix of red and green light stimulates both cones somewhat evenly for an overall perception of yellow. Other combinations produce different colors – red and blue mix to violet, green and blue to cyan, and red, green, and blue together produce white light.
Why Yellow and Not Orange?
You may wonder why combining red and green light appears yellowish instead of orange. Orange light has a dominant wavelength around 610nm, between red and green. However, when red (~700nm) and green (~550nm) mix, there is no specific orange stimulation. The red-green response is broadly spread across the visible spectrum, which is perceived as yellow.
In the opponent process theory of color vision, cells receiving combined red+green input are stimulated maximally by yellow wavelengths around 575nm. So the balance of red and green elicits a strong yellow perception rather than orange.
Pigment Mixing vs. Light Mixing
Importantly, mixing red and green pigments or dyes yields a different result than mixing red and green light. Pigments selectively absorb certain wavelengths and reflect the rest. When red and green paints mix, the reflected light lacks both red and green components, leaving a dull brown.
Mixing color lights directly stimulates red and green cones additively. The brain’s opponent channels process this combined response as yellow. So mixing light is additive, while mixing pigments is subtractive. Red + green light = yellow light. But red + green paint = brown paint.
Applications of Additive Color
Understanding that mixing colored lights produces different hues than material pigments allows us to engineer systems that exploit additive color properties. Some key applications include:
- Computer/TV monitors – Red, green, and blue phosphor dots additively mix to produce on-screen colors.
- Digital projectors – Combining the primary RGB colors of projected light creates the full spectrum on screens.
- Laser light shows – Beams with different wavelengths can mix new colors for dazzling displays.
- LED lighting – Mixing red, green, and blue LEDs allows creating customizable tinted light.
In these technologies, controlling the intensity of the red, green, and blue sources allows selectively stimulating the eye’s cones to produce any color sensation we desire. Mixing lights brings far greater versatility than mixing pigments.
The Special Case of Magenta
Unlike mixing red and green to yield yellow, combining wavelengths of red and blue light elicits a unique color called magenta. Magenta does not exist as a single wavelength on the visible spectrum – there is no beam of magenta light. It is a non-spectral color.
Magenta results when the red and blue cones are stimulated fairly equally, without much green cone response. Since green is the opponent color to magenta, suppressing green allows the red-blue perception to dominate. The resulting magenta color lies outside the boundaries of the real color spectrum.
Colorblindness Alters Mixtures
It is also worth noting that colorblind individuals will perceive the mixture of red and green light differently than those with normal color vision. Those with red-green color blindness lack functional red or green cones, causing an inability to distinguish these colors.
As a result, mixing red and green will not appear yellow for colorblind observers, since their cones cannot normally respond to these wavelengths. The exact perception will depend on which cone type is missing. So color mixing has a unique appearance for each individual depending on the functioning of their photoreceptors.
Conclusion
In summary, combining wavelengths of red and green light elicits a perception of yellow in normal human vision. This additive mixing stimulates both red and green cones in the retina fairly evenly, with yellow as the opponent color between red and green on the visible spectrum. Understanding additive color and the eye’s trichromatic system helps explain the difference between mixing lights and pigments. Engineers utilize these principles to enable full-color displays and designs in various technologies.
