Human perception of color is limited by our biological makeup. The spectrum of light visible to humans represents only a small segment of the vast electromagnetic spectrum. This means that many “colors” exist beyond our visual capabilities.
How Humans See Color
Human color perception begins in the retina, a light-sensitive layer at the back of the eye. Within the retina are specialized photoreceptor cells: rods and cones. Rods are primarily responsible for vision in low light conditions, detecting shades of gray rather than color. Cones, on the other hand, are responsible for our ability to perceive color in brighter light.
Humans typically possess three types of cone cells, sensitive to long (reddish), medium (greenish), and short (bluish) wavelengths of light. The brain processes the varying signals from these three cone types to interpret the colors we experience. This trichromatic vision system defines the boundaries of our natural color world.
The Visible Light Spectrum
Light is a form of electromagnetic radiation, which travels in waves and encompasses a broad range of wavelengths. The visible light spectrum is the narrow portion of this radiation that the human eye can detect. This range typically spans from approximately 380 nanometers (nm) to about 700 nanometers.
Within this visible range, different wavelengths correspond to different colors, creating the familiar sequence of red, orange, yellow, green, blue, indigo, and violet (ROYGBIV). Wavelengths shorter than violet are considered ultraviolet, while those longer than red are infrared.
Colors Beyond Human Perception
Beyond the familiar rainbow of visible light lie wavelengths imperceptible to the human eye, primarily ultraviolet (UV) and infrared (IR) light. Our cone cells are not equipped to detect these wavelengths. Ultraviolet light, with wavelengths shorter than visible violet light, ranges from approximately 10 to 400 nanometers. Sources of UV light include sunlight and specialized lamps.
Infrared light, conversely, has wavelengths longer than visible red light, typically ranging from about 750 nanometers up to 1 millimeter. Though we cannot see it, we often sense infrared radiation as heat, such as the warmth from the sun or a heat lamp. Everyday devices like television remote controls also utilize infrared signals to communicate.
Detecting the Unseen
Despite our inherent inability to see ultraviolet and infrared light, humans have developed technologies to detect and even visualize these forms of radiation. Thermal cameras, for instance, are designed to capture infrared energy, which all objects emit as heat. These cameras convert the varying heat signatures into a visible image, often displayed with different colors representing temperature differences. This allows humans to “see” heat, which is particularly useful in applications like identifying heat loss in buildings or locating objects in complete darkness.
Night vision goggles offer another example of technology extending human perception. These devices work by amplifying extremely small amounts of existing light that are otherwise insufficient for human vision. The amplified light is then converted into a visible image, allowing us to navigate and observe in near-total darkness. These tools translate invisible wavelengths into a format our eyes can interpret.
Animal Vision and Invisible Colors
The limitations of human vision become particularly clear when examining the diverse visual capabilities of other species. Bees, for example, have a different color perception system than humans, seeing ultraviolet (UV) light in addition to blue and green. They cannot see red light, but their UV vision allows them to see intricate patterns on flowers, known as nectar guides. These patterns direct bees towards nectar and pollen, demonstrating how different visual adaptations serve specific ecological roles.
Certain snakes, such as pit vipers, boas, and pythons, possess specialized organs called pit organs that enable them to detect infrared thermal radiation. These pit organs act as highly sensitive heat detectors, allowing the snakes to create a “thermal image” of their surroundings. This unique sensory ability allows them to accurately locate warm-blooded prey, even in complete darkness.