The world we perceive as vibrantly colored is only a small fraction of a much larger reality. Color is not an inherent property of an object, but rather a sensation created in the brain when light energy interacts with our eyes. This light energy exists as electromagnetic radiation, traveling in waves of varying lengths. Because human sensory organs are finely tuned to a particular slice of this spectrum, the vast majority of light waves remain invisible to us.
Defining the Visible Spectrum
The light that humans can perceive is known as the visible spectrum, a narrow band of electromagnetic waves defined by their length. These wavelengths are measured in nanometers (nm), and the human eye generally registers light from approximately 380 nm to 750 nm. The colors we know—from violet to red—correspond directly to this specific range of wavelengths. Violet light has the shortest wavelength and highest energy, while red light has the longest wavelength and lowest energy.
The full electromagnetic spectrum includes everything from radio waves, which can be kilometers long, to gamma rays, which are smaller than an atom. The visible band occupies a tiny space between the higher-energy ultraviolet and the lower-energy infrared regions. This particular range is not accidental; it largely corresponds to the radiation that passes most efficiently through both the sun and Earth’s atmosphere. Our visual system therefore evolved to utilize the most abundant and accessible light available on the planet’s surface.
The Invisible Spectrum Below Red
Immediately following the longest visible red wavelengths is the invisible region known as infrared (IR) radiation. This radiation begins at approximately 750 nanometers and extends up to about 1 millimeter. All objects that possess thermal energy emit infrared radiation. This means we are constantly bathed in a form of light that is a direct measure of heat.
Because of this property, infrared is not sensed by humans as a color but rather as warmth on the skin. Near-infrared waves, those closest to the visible red light, are used in applications like remote controls and fiber-optic communication. Longer infrared waves, such as those used in thermal imaging, allow specialized cameras to create pictures by detecting the heat emitted by objects. These thermal cameras translate invisible temperature variations into visible color gradients, effectively allowing us to “see” heat signatures.
The Invisible Spectrum Beyond Violet
At the opposite end of the visible range, preceding the shortest wavelength of violet light, lies ultraviolet (UV) radiation. This high-energy light has wavelengths shorter than 400 nanometers, extending down to about 10 nanometers. The higher frequency of UV photons means they carry significantly more energy than visible light, enabling them to interact with and alter organic molecules.
This heightened energy is why excessive UV exposure can be damaging to human cells, leading to sunburn and an increased risk of skin cancer. Despite this, UV light is used beneficially in many applications due to its powerful properties. Short-wave UV-C radiation is germicidal, used to sterilize medical equipment and purify water by destroying the DNA of microorganisms. Long-wave UV-A light is used in tanning beds and black lights, causing certain materials to fluoresce, or glow, as energy is re-emitted as visible light.
Biological Constraints and Seeing More Color
The boundary of the visible spectrum is dictated by the biological structure of the human eye. Our ability to perceive color is based on having three types of cone cells in the retina, a condition known as trichromacy. Each cone type is sensitive to a different range of wavelengths, broadly corresponding to short (blue), medium (green), and long (red) light. The brain interprets the relative signals from these three types to construct the million or so colors we can distinguish.
This three-channel system imposes a limit on our color experience, making certain hues like “red-green” or “yellow-blue” impossible to perceive simultaneously. Under specific neurological circumstances, such as prolonged staring at a color followed by looking at a neutral surface, a phenomenon known as “chimeric colors” can occur. These colors are generated in the brain by fatiguing the cone cells.
Many species, including birds, fish, and insects, possess four types of cone cells, making them tetrachromats. This additional cone often extends their vision into the ultraviolet range, allowing them to see wavelengths that are invisible to humans. For example, birds may use UV light to discern patterns on feathers or find nectar guides on flowers that appear plain to us. This expanded sensory capacity means that tetrachromatic animals likely perceive a much richer color space, potentially distinguishing up to 100 million different hues.
Research suggests that some women may also be conditional tetrachromats, carrying a genetic variation that gives them a fourth cone type. While having the extra cone is necessary, the brain must also be able to process the additional color information for true tetrachromacy to be realized. This potential for enhanced color vision in a small percentage of the human population demonstrates that our current color palette is more of a biological constraint than a physical limit of light itself.