The color blue can sometimes appear to have purple tones or hues. This can happen for several different reasons relating to how our eyes perceive color, as well as environmental factors that can alter the way blue light is reflected. Understanding what makes blue look purple involves delving into the physics and biology behind human color vision.
How Human Color Vision Works
Human color perception stems from the retina, a light-sensitive layer of tissue at the back of the eye. The retina contains photoreceptor cells called rods and cones. Rods are sensitive to light and dark, but do not detect color. Cones detect color and are responsible for our ability to discern different hues.
There are three types of cones, each containing pigments that are sensitive to different wavelengths of light:
- S cones – sensitive to short blue wavelengths of light
- M cones – sensitive to medium green wavelengths
- L cones – sensitive to long red wavelengths
| Cone Type | Light Wavelength Detected |
|---|---|
| S cones | Short blues |
| M cones | Medium greens |
| L cones | Long reds |
When light enters the eye and hits the retina, it stimulates the cones to different degrees depending on the wavelength. The brain interprets the signals from the three cone types to produce the perception of color. For example, blue light strongly stimulates the S cones, weakly stimulates the M cones, and minimally stimulates the L cones. This cone stimulation pattern is interpreted by the brain as the color blue.
The Visible Light Spectrum
Different hues correspond to different wavelengths of visible light. The visible light spectrum ranges from violet with the shortest wavelengths to red with the longest:
| Color | Wavelength Range (nm) |
|---|---|
| Violet | 380-450 |
| Blue | 450-495 |
| Green | 495-570 |
| Yellow | 570-590 |
| Orange | 590-620 |
| Red | 620-750 |
Blue wavelengths range from about 450-495 nanometers (nm). Purple is not actually present in the visible spectrum – it exists in our minds as a mix of blue and red. Wavelengths from about 400-450 nm stimulate both the S cones and the L cones, and this is perceived as purple.
So for blue light to take on a purple hue, it must shift towards these shorter wavelengths that stimulate both blue and red cones. There are a few reasons this color shift can happen.
Factors That Make Blue Look Purple
There are several environmental and perceptual factors that can cause blue to appear more violet or purple:
Dim Lighting
In dim conditions, the S cones become less sensitive. The weak stimulation of S cones combined with the longer wavelengths reflecting off blue objects causes them to activate more L cones. This makes the blue take on a purplish red tint.
Color Contrast
The contrast effect describes how the presence of complementary colors influences how we perceive a color. Complementary colors are opposite each other on the color wheel. The complement of blue is orange.
Viewing blue alongside orange makes the blue appear more violet because of the enhanced stimulation of L cones processing the orange wavelengths. The orange color contrast brings out the hint of redness in blue needed for it to look purple.
Optical Illusions
Certain optical illusion images are specifically designed to trick our eyes into seeing blue as purple. These illusions manipulate color contrast, shadows, and other effects to shift blue hues towards violet.
Well-known examples include thespirial illusion and the concentric circles illusion. In both illusions, the blue areas seem to take on a purple tone due to contrast and distorted edges.
| Optical Illusion | Image |
|---|---|
| Spiral illusion |  |
| Concentric circles illusion |  |
Color Blindness
Some types of color blindness can also cause blue to look purple or violet. The most common is a red-green color vision deficiency where people have trouble distinguishing red and green hues.
In these cases, blue wavelengths that normally only stimulate S cones will also stimulate some L cones. This makes the blue take on a purple appearance.
Aging and Cataracts
As people age, the lenses in their eyes start to yellow and cloud, leading to cataracts. These changes filter out some of the short wavelength blue light before it reaches the retina.
The reduced stimulation of S cones makes blue look less saturated. With more long wavelength light reflecting off blue objects, they stimulate more L cones and can take on a violet/purple hue.
The Purplish Tint of Blue Skies
An interesting real-world example of normal blue taking on purple tones can be seen in the changing sky colors at dusk and dawn.
During the day, the sky appears blue due to the shorter 450 nm wavelengths scattering more as sunlight passes through the atmosphere. But at sunrise and sunset, the low angle of the sun means its light has to pass through more air. This scatters out even more shorter blue wavelengths.
With less stimulation of S cones but still some long wavelength light reaching our eyes, the morning/evening sky takes on an intriguing purplish hue.
Key Factors Behind the Blue-Purple Shift
In summary, the key factors that can cause blue colors to appear more violet or purple include:
- Dim lighting – less S cone stimulation
- Adjacent warm color contrasts – enhanced L cone stimulation
- Optical illusions – exploits color contrast effects
- Color blindness – anomalous stimulation of L cones
- Aging/cataracts – filtering of short blue wavelengths
- Scattering of sunlight – removes more short blue light
Understanding these various physiological and optical effects gives us insight into the subjective nature of human color perception. While we may see blue and purple as distinct hues, our eyes and brain can sometimes blur the lines through complex processing.
Conclusion
Blue takes on purple or violet characteristics when there is reduced stimulation of S cones and/or enhanced stimulation of L cones. This cone activity shift skews blue colors toward the 400-450 nm range that is perceived as purple. Factors like dim lighting, adjacent colors, optical illusions, and changes in the eye all play a role in making blue appear less saturated and take on the red hint needed for a purple hue. So in the end, blue looking purple comes down to the complexities and quirks of human vision.
