Color mixing is a fascinating topic in physics and art. When different colored lights mix together, the results can sometimes be counterintuitive. We’re all familiar with the vivid purple that results when blue and red paint are mixed together. But does the same thing happen when blue and red light mix? In this article, we’ll explore the physics of light and color to find out if combining beams of blue and red light truly makes the color purple.
Primary Colors of Light vs. Paint
To understand what happens when different colors of light mix, we first need to talk about the primary colors of light. In paint and other pigments, the primary colors are red, blue and yellow. This means that all other colors can be made by mixing combinations of these three. Light works differently though – the primary colors of light are red, green and blue. This is because of the way our eyes perceive color through specialized receptor cells called cones.
The three types of cones in our eyes are stimulated to different degrees by different wavelengths of light. Red cones are most sensitive to long wavelengths, green cones to medium wavelengths, and blue cones to short wavelengths. Every visible color stimulates at least one of these cone types. Mixing red, green and blue light together in different proportions can therefore recreate any color that the human eye can perceive.
Additive vs. Subtractive Color Mixing
So why are the primary colors different for light and pigments? It comes down to the physics of how color is created.
With pigments and dyes, color mixing is subtractive. These materials absorb certain wavelengths of light and reflect the rest. For example, a red paint pigment absorbs green and blue light, and reflects mainly red. As more pigments are mixed together, more wavelengths are subtracted from white light, eventually resulting in black if all colors are mixed.
Light mixing, on the other hand, is additive. Instead of absorbing wavelengths, colored light sources directly emit specific wavelengths. Red, green and blue light beams contain no overlap in wavelengths. Mixing these adds more wavelengths, eventually combining to white light if all three primary colors are mixed equally.
Mixing Red and Blue Light
Now we can address our original question – does mixing red and blue light make purple? Based on the additive color properties described above, we would expect the combination of narrow bands of red and blue light to look like a light purple color. However, in practice it ends up looking different than most people expect.
Let’s try a little experiment. Imagine two flashlights – one with a red LED bulb and one with a blue LED bulb. In a dark room, shine the red light on one wall and the blue light on an adjacent wall. Where the two beams overlap there will be a zone of magenta-colored light.
| Color | Wavelength range |
|---|---|
| Red | 620-750 nm |
| Blue | 435-500 nm |
Magenta is a reddish purple hue. But most people associate pure purple with mixing blue and red paint pigments. Why doesn’t shining blue and red light together make the same violet shade?
Perception of Magenta Light
The reason that combining blue and red light appears magenta relates to the tricks our visual system plays to create the perception of color. Remember that the primary colors of light — red, green and blue — correspond with the three types of color-sensitive cones in our eyes.
Magenta light, comprising only red and blue wavelengths, doesn’t actually stimulate the green cones at all. In reality, magenta is not contained within the visible color spectrum! Our eyes and brain fill in the missing green stimulation with a constructed color that seems to lie between the red and blue.
In color theory, magenta is called a non-spectral color. Other non-spectral colors include purple and white. We perceive them only because our visual system interpolates between the stimulus of two or more actual spectral colors in the light spectrum.
True Violet Light
This raises the question – is there a way to create true violet light by combining wavelengths?
Violet is a spectral color, meaning it corresponds to a real wavelength range in the visible light spectrum. The violet wavelengths range from about 380-450 nm, which is shorter than blue and overlaps somewhat with indigo light.
To make violet light through additive color mixing, we need to combine a shorter wavelength blue light with more of a true red wavelength around 700 nm. An LED spotlight with these specifications, shone together with a red spotlight, would produce the violet coloring that we expect from mixing red and blue pigments.
Computer and TV screens provide another example of additive violet light. These displays use red, green and blue pixels to create colors. Lighting up both the blue and red pixels together displays a vivid violet color. Modern screens can reproduce almost any hue by mixing different levels of the three primary colors of light.
Pigment vs. Light Mixing Summary
While red and blue paint predictably mix to make purple, the same is not true when combining red and blue light. Because of the quirks of human vision and color perception, shining red light and blue light together produces magenta, which is a distinctly different color than pigment-based purple. True violet light can only be created through careful selection of red and blue wavelengths in the additive color mixing process.
The takeaway is that the primary colors and mixing principles for pigments vs. light are very different. This stems from the subtractive vs. additive physics underlying how color is produced with each. So don’t be surprised if mixing two colors of light doesn’t give the same result as mixing corresponding paints!
Other Interesting Mixes of Light Color
The red and blue combination is just the start of investigating additive color mixing. Combining other pure hues of light leads to additional counterintuitive results. Here are some other examples:
- Green + Red light = Yellow. This is because when all wavelengths except blue are present, we see yellow.
- Green + Blue light = Cyan (blue-green). Cyan lacks red wavelengths.
- Red + Green + Blue light = White. All wavelengths together make white light.
Understanding these sometimes non-intuitive results requires digging into concepts in physics, visual perception, color theory and psychology. But the takeaway is that mixing colors of light follows very different rules than mixing pigmented colors.
Applications of Additive Color Mixing
The principles of additive color have many high-tech applications today. LCD and OLED screens mix red, green and blue light to create sharp, brilliant displays. Full color LED lighting systems can produce any hue. Stage lighting rigs allow intricate mixes of color using red, green, blue and other LEDs. Even laser light shows work by scanning and mixing different laser wavelengths.
Conclusion
Does combining blue and red light make purple? While mixing blue and red pigments makes a purple hue as expected, the same is not true for light. Because of the physics of light and color vision, blue and red light mix to create magenta. True violet light can only be produced by careful selection of blue and red wavelengths. This reveals some fascinating aspects of visual perception and reminds us that with light, color mixing follows an entirely different set of rules compared to pigments.
