A cyan colored object appears blue-green to the human eye. But what colors of light is it actually absorbing and reflecting to produce this cyan appearance? The answer lies in understanding how color perception works.
The visible light spectrum that humans can see consists of wavelengths ranging from about 400 to 700 nanometers. The longest wavelengths around 700 nm are perceived by our eyes as the color red. As the wavelengths get shorter, they appear as orange, yellow, green, blue, indigo and finally violet at around 400 nm.
White light contains a full spectrum of wavelengths. When an object absorbs some wavelengths more than others, the wavelengths that are reflected or transmitted give the object its perceived color.
How the Eye Perceives Color
The human eye has photoreceptor cells called cones that are sensitive to different wavelengths of light. There are three types of cones:
- S cones – Sensitive to short blue wavelengths
- M cones – Sensitive to medium green wavelengths
- L cones – Sensitive to long red wavelengths
When light enters the eye, it stimulates each of these cone types to varying degrees depending on its wavelengths. The relative stimulation of the three cone types is interpreted by the brain as a specific color.
For example, red light with long wavelengths around 700nm strongly stimulates the L cones but provides little stimulation to the S and M cones. This stimulation pattern is perceived as the color red. Blue-green light provides strong stimulation to the S cones and less to the L and M cones, which is perceived as cyan.
Cyan Color Perception
Cyan is a combination of green and blue light. Cyan light stimulates the S cones strongly, the M cones moderately, and stimulates L cones very little.
Specifically, cyan light spans a range of wavelengths from about 490 to 520 nm. This stimulates the green M cones and blue S cones significantly, while providing minimal stimulation to the red L cones.
The brain interprets this S and M cone stimulation along with very little L cone stimulation as the color cyan.
Light Absorption of a Cyan Object
For an object to appear cyan, it must absorb most of the longer orange-red wavelengths while reflecting the green and blue wavelengths.
As white light shines on the object, it absorbs the long red wavelengths strongly but reflects back the shorter blue and green light. This gives it a cyan appearance.
To visualize this, here is a table showing the approximate light absorption and reflection properties of a cyan-colored object:
| Wavelength (nm) | Color | Absorption | Reflection |
|---|---|---|---|
| 700-400 | Red | Strong | Weak |
| 590-500 | Green | Moderate | Strong |
| 490-450 | Blue | Weak | Strong |
As seen above, a cyan object absorbs long red wavelengths strongly while reflecting back middle green and short blue wavelengths. This selective absorption and reflection of parts of the visible spectrum results in the light being filtered to just the blue-green wavelengths that give cyan its color.
Why Cyan Appears Blue-Green
Cyan is perceived as a blue-green color because it stimulates the eye’s S and M cones about equally. The S cones are most sensitive to blue light while the M cones respond strongly to green light. Stimulating both approximately equally leads to the blending of blue and green that is seen as cyan.
If an object were to reflect just the shortest blue wavelengths around 450-500 nm, it would appear deep blue since only the blue-sensitive S cones would be strongly stimulated. If it reflected just the middle green wavelengths around 500-560 nm, it would appear green because those wavelengths would mainly stimulate the green-sensitive M cones.
But because cyan reflects both the blue and green wavelengths, the brain blends those color perceptions together and sees cyan as a vibrant blue-green color.
Cyan Dye and Pigment Colors
When creating cyan colors with dyes and pigments, there are two approaches:
- Mixing a blue dye/pigment with a green dye/pigment
- Using a cyan dye/pigment that inherently reflects blue and green wavelengths
With the first approach, combining a blue that reflects short wavelengths and a green that reflects middle wavelengths yields a cyan blend. This is like mixing blue and green paint.
The second approach uses a single dye or pigment formulated to reflect both blue and green light while absorbing orange-red light. This inherently cyan material filters white light to produce the blue-green color.
Examples of inherent cyan pigments include phthalocyanine blue and cyanine blue. These contain specialized structures ideal for reflecting the short and middle wavelengths that produce a cyan color.
Cyan Filter
An interesting way to visualize cyan light absorption is with a cyan filter. This is a filter material such as plastic or glass that preferentially absorbs orange-red wavelengths while allowing blue and green wavelengths to pass through.
When white light shines through the filter, it selectively absorbs the long red wavelengths. The short blue and middle green wavelengths pass through unabsorbed, producing cyan transmitted light.
Here is how a cyan filter changes the balance of wavelengths in white light:
| Wavelength | White Light | Transmitted Light |
|---|---|---|
| 700-400 nm (Red) | Strong | Weak |
| 590-500 nm (Green) | Strong | Strong |
| 490-450 nm (Blue) | Strong | Strong |
As shown above, the cyan filter absorbs most of the long red light while allowing blue and green wavelengths to pass. This filtering action produces cyan transmitted light, demonstrating the selective light absorption properties of cyan materials.
Applications Using Cyan Light Absorption
The unique light absorption characteristics of cyan materials lend them to specialized color applications:
- Cyan printing inks – Cyan inks absorb orange-red wavelengths, allowing blue-green hues to reflect back from the paper.
- Cyan photographic filters – Absorb red light, giving landscape photographs increased blue-green coloring.
- Cyan dyes in textiles – Produce blue-green shades on fabrics and clothing.
- Cyan in color monitors – Along with magenta and yellow pixels, cyan pixels filter white backlight to produce on-screen colors.
In all cases, cyan absorbs the warmer orange-red wavelengths while selectively reflecting back the cooler blue-green light – the hallmark behavior of cyan color perception.
Conclusion
In summary, a cyan colored object preferentially absorbs light at the long orange-red wavelengths from around 700 to 590 nm. It reflects back light at the short blue wavelengths from 490 to 450 nm and middle green wavelengths from 590 to 500 nm. This selective absorption and reflection filters white light to just the blue-green wavelengths that elicit a cyan color perception in the human eye. So a cyan object does indeed strongly absorb red light while reflecting the blue and green light that gives it its cool blue-green color.