Color, a fundamental part of human experience, is our perception of a small segment of the vast electromagnetic spectrum. The electromagnetic spectrum encompasses various forms of energy, from radio waves to X-rays, all traveling in waves. Human eyes are only capable of detecting a narrow band of these wavelengths, which we define as visible light.
How Human Vision Works
Human vision relies on specialized photoreceptor cells within the retina. There are two primary types: rods and cones.
Rods are highly sensitive to light intensity, allowing us to see in dim conditions, but they do not perceive color, rendering vision in shades of gray. Cones are responsible for color vision and operate best in brighter light. Most people possess three types of cones, each sensitive to different wavelengths: short-wavelength (blue), medium-wavelength (green), and long-wavelength (red) light. When light strikes these cones, they send signals to the brain. The brain interprets these signals to create our perception of millions of distinct colors. The visible spectrum, typically spanning wavelengths from about 380 to 700 nanometers, activates these cones.
Light Beyond Our Sight
Beyond the visible spectrum lie forms of light humans cannot perceive directly, such as ultraviolet (UV) and infrared (IR) radiation. Ultraviolet light has shorter wavelengths than visible light, ranging from about 10 to 400 nanometers. This invisible light is categorized into UVA, UVB, and UVC, with UVC and most UVB largely filtered by Earth’s atmosphere. The sun is a natural source of UV radiation, also produced by artificial sources like black lights and specialized lamps used for sterilization due to its ability to damage microorganisms.
Infrared light possesses longer wavelengths than visible light, extending from around 700 nanometers up to 1 millimeter. This radiation is often associated with heat, as all objects with a temperature above absolute zero emit infrared energy. Common examples include the warmth felt from a campfire or a heated object, the signals from a television remote control, and the thermal energy emitted by the human body. Infrared light is utilized in various applications, such as night vision equipment and thermal imaging cameras, which detect heat signatures to form images in darkness.
Why Our Eyes Have Limits
The specific range of light our eyes can detect is a result of biological and evolutionary adaptations. Our vision evolved to perceive the wavelengths most abundant and useful in our environment.
Earth’s atmosphere and water also act as filters, allowing visible light to pass through efficiently while absorbing or scattering much of the other electromagnetic radiation. The photoreceptors in our eyes contain specific photopigments that are chemically tuned to respond only to the wavelengths within the visible spectrum. While other wavelengths carry energy, their interaction with these pigments is either insufficient to trigger a signal or could potentially cause damage, especially in the case of high-energy UV light. This evolutionary fine-tuning optimizes human vision for our terrestrial environment.
Detecting the Invisible
While humans cannot naturally see ultraviolet or infrared light, scientific advancements have allowed us to develop technologies that make these invisible wavelengths detectable. Night vision goggles capture ambient infrared radiation and convert it into a visible image, enabling sight in near-total darkness. Similarly, UV lamps are used for various purposes, from authenticating currency to sterilizing surfaces, by emitting ultraviolet light that interacts with specific substances.
Many animals possess visual capabilities that extend beyond the human visible spectrum. Bees and some birds can perceive ultraviolet light, which helps them locate nectar in flowers or identify patterns on feathers invisible to human eyes. Certain snakes, such as pit vipers, have specialized pit organs that detect infrared radiation, allowing them to sense the body heat of their prey in complete darkness. Some fish, like goldfish and salmon, can also see into the infrared range, aiding their navigation and hunting in aquatic environments.