The mixing of the colors red and blue to create purple is a concept we are all familiar with from a young age. But what is actually happening on a scientific level when these two colors combine to form a new one? In this article, we will explore the physics and chemistry behind red, blue, and purple light, and reveal what makes these primary colors blend to create the satisfying hue that is purple.
The Physics of Color
To understand what makes purple, we first need to understand a bit about the physics of color and light. Visible light consists of a spectrum of different wavelengths of electromagnetic radiation. The specific wavelength of light determines what color our eyes perceive it as.
Red light has the longest visible wavelengths, ranging from around 700 nanometers to 620 nm. Blue light has shorter wavelengths ranging from around 500 nm down to 450 nm. When red and blue light mix together and enter our eyes at the same time, our visual system interprets the combined wavelengths as the color purple.
Differences in Wavelengths
The reason red and blue make purple when combined has to do with the way our eyes detect color. We have special photoreceptor cells in our retinas called cone cells that are sensitive to different wavelengths of light. There are three types of cone cells, each tuned to be most sensitive to long, medium or short wavelength light.
| Cone Cell Type | Peak Sensitivity |
|---|---|
| S-cones (short) | 420-440 nm (blue) |
| M-cones (medium) | 534–545 nm (green) |
| L-cones (long) | 564–580 nm (red) |
When red light (around 700 nm) and blue light (around 450 nm) hit our retinas simultaneously, the red light stimulates the L-cones most strongly, while the blue light stimulates the S-cones. Our visual system combines and interprets these signals as the new color purple.
Additive Color Mixing
The mixing of red and blue light to create purple is an example of additive color mixing. With additive mixing, wavelengths of light combine to excite multiple cone cell types, producing the perception of new colors. This is different from subtractive color mixing where pigments absorb certain wavelengths selectively.
Additive color mixing using red, blue and green light is the basis for many color technologies including TVs, computer monitors, projectors and more. These devices produce a wide range of colors by combining fixed amounts of red, blue and green light. Not coincidentally, purple is one of the colors produced by combining emissions from the red and blue channels.
Overlapping Sensitivity Curves
Looking at the sensitivity curves for the different cone cells provides further insight into how red and blue make purple. As the graph below illustrates, L-cones and S-cones have overlapping regions of sensitivity. Red light excites L-cones but not S-cones. Blue light excites S-cones but not L-cones. Purple light of the right wavelength and intensity excites both L- and S- cones to a similar degree. Our visual system interprets this combined neural signal as the color purple.
The Role of M-cones
While purple light strongly stimulates L- and S-cone cells, it stimulates M-cone cells to a lesser extent. The M-cones have a sensitivity peak between the L- and S- cones. They are moderately stimulated by both red and blue light, just not as strongly as the L- and S-cones, respectively. The M-cone signals contribute to the perception of purple as well, making it distinct from pure red or blue light.
Varying Shades of Purple
By adjusting the intensity and exact wavelengths of red and blue light that mix, it is possible to produce a wide range of purples, from reddish purples to blueish purples. More intense red light shifts the mix toward reddish purple hues, while more intense blue light shifts it toward blueish purples. Fine tuning the stimulation of L-, M- and S-cones allows purples across the spectrum to be produced through additive red + blue mixing.
Pigment and Dye Mixing
When mixing colored pigments or dyes, such as paints or inks, combining red and blue also produces purple. But the physics of subtractive color mixing is different than additive light mixing. Pigments selectively absorb certain wavelengths of light and reflect others. A purple pigment absorbs greens, yellows and oranges, while reflecting reds and blues. But at the atomic level, similar principles of light interaction are at play when mixing pigmented or dyed reds and blues to create purples.
Light Perception and the Brain
Beyond the retina, signals from L-, M- and S-cone cells travel via the optic nerve to the brain where additional color processing occurs. Visual area V1 receives simple wavelength signals then more complex color processing happens in extrastriate areas. The end result is our perceptual experience of a new color called purple when red and blue mix. This demonstrates the importance of both optical reception and neural processing in the phenomenon of red and blue combining to make purple.
Conclusion
When red and blue light mix, the red wavelengths strongly stimulate L-cone cells in our retinas, while the blue wavelengths stimulate S-cone cells. The combined signals are interpreted by the visual system as a new color: purple. This additive mixing allows a range of purples to be created by adjusting the respective intensities of red and blue light. A similar principle operates at the molecular level when red and blue pigments or dyes mix to create purple through subtractive color mixing. So while purple is a simple sensory experience, the science behind red and blue making purple is fascinatingly complex.
