The world we perceive through our eyes, rich with a spectrum of colors, represents only a fraction of the electromagnetic radiation surrounding us. Human vision operates within a limited range of wavelengths. This raises a compelling question: are there colors that exist beyond our perception? Exploring the unseen parts of the light spectrum reveals a reality far more expansive than what our everyday senses can detect.
Our Visible World
Color, from a scientific standpoint, is our brain’s interpretation of different wavelengths of light within the electromagnetic spectrum. This vast spectrum includes everything from radio waves to gamma rays, but only a narrow band is visible to the human eye. This visible light typically ranges from approximately 380 to 750 nanometers (nm). Within this range, shorter wavelengths correspond to violet and blue, while longer wavelengths appear as red.
Our eyes contain specialized photoreceptor cells called cones, which detect various light wavelengths and translate them into the colors we perceive. Humans possess three types of cone cells, each sensitive to different parts of the visible spectrum—one for short wavelengths (blue), one for medium (green), and one for long (red). The combined signals from these cones allow us to experience the myriad of colors in our environment.
Unseen Colors: Ultraviolet
Beyond the violet end of the visible spectrum lies ultraviolet (UV) light, with wavelengths shorter than those visible to humans (10 to 400 nm). The human eye is generally unable to perceive UV light because the eye’s lens absorbs most of it before it can reach the retina.
Many animals possess the ability to see in the ultraviolet range. Bees, for instance, use UV vision to identify intricate patterns on flowers that guide them to nectar. Birds also utilize UV perception, which helps them in mate selection by revealing hidden UV-reflective plumage patterns. Some mammals, including cats, dogs, pigs, and reindeer, can also detect UV light, aiding them in tasks like locating food or detecting predators.
Unseen Colors: Infrared
On the opposite end of the spectrum, beyond red, is infrared (IR) light, with longer wavelengths than visible light (700 nm to one millimeter). Human eyes cannot perceive IR light because its photons possess insufficient energy to trigger the photoreceptors in our retinas. The water content within our eyeballs also absorbs infrared radiation, preventing it from reaching the light-sensitive cells.
Several animal species have evolved mechanisms to detect infrared light. Pit vipers, boas, and pythons, for example, possess specialized pit organs that function as heat sensors, allowing them to locate warm-blooded prey by sensing their emitted infrared radiation. Some insects, like mosquitoes, also use IR detection to find hosts by sensing body heat. While most mammals cannot see infrared, some researchers suggest that animals like foxes may have some sensitivity to these wavelengths, though direct visual perception is rare for warm-blooded creatures.
Why Our Vision is Limited
The limitations of human vision stem from biological and evolutionary factors. The human retina contains two main types of photoreceptors: rods and cones. Rods are highly sensitive to low light levels and are responsible for night vision, but they do not detect color. Cones require brighter light and are responsible for color vision. The sensitivity of these photoreceptors, along with the filtering properties of the human lens, defines our visible spectrum.
From an evolutionary perspective, human vision developed to be optimized for conditions prevalent on Earth’s surface. The sun emits a significant portion of its energy in the visible light range, and this light passes relatively unimpeded through Earth’s atmosphere. Visible light also interacts effectively with objects in our environment, providing information about their shape, color, and movement. This was advantageous for our ancestors in identifying food sources, navigating terrain, and detecting threats. The abundance and clarity of visible light made it the most advantageous spectrum for human eyes to evolve within.