Visible spectrum

Based on Wikipedia: Visible spectrum

Isaac Newton thought seven was a perfect number. That's why you learned the rainbow has seven colors in school—not because of physics, but because a seventeenth-century genius was influenced by ancient Greek mysticism about the connection between colors, musical notes, and the days of the week. The color indigo, wedged awkwardly between blue and violet, exists in our cultural memory mainly because Newton wanted his spectrum to match his cosmological beliefs.

This is the story of how we came to understand the narrow slice of electromagnetic radiation that our eyes can detect—and how that understanding reveals as much about the quirks of human perception as it does about light itself.

A Ribbon of Light in an Invisible Ocean

The electromagnetic spectrum is vast. Radio waves can be longer than a football field. Gamma rays are smaller than an atomic nucleus. Somewhere in this enormous range—spanning wavelengths that differ by a factor of about ten trillion—there's a tiny window where radiation happens to excite the photoreceptors in human eyes.

That window runs from roughly 380 to 750 nanometers.

A nanometer is one billionth of a meter, so we're talking about wavelengths roughly four hundred to seven hundred times smaller than the width of a human hair. This is visible light: the only electromagnetic radiation you can perceive directly without instruments. Everything else—radio, microwave, infrared, ultraviolet, X-ray, gamma ray—passes through or around you invisibly.

The boundaries aren't sharp. Under ideal laboratory conditions, some people can perceive infrared light up to about 1,064 nanometers, and young subjects with pristine lenses can detect ultraviolet down to 310 nanometers. But for practical purposes, 380 to 750 nanometers captures what most humans can see most of the time.

Here's what's remarkable: this range isn't arbitrary. It corresponds almost perfectly to the "optical window" in Earth's atmosphere—the wavelengths that pass through air largely unimpeded. Evolution tuned our eyes to the light that was actually available. The sun floods our planet with electromagnetic radiation, but the atmosphere filters much of it out. Our visual system evolved to exploit the radiation that made it through.

Newton's Prism and the Corpuscles of Color

Before Newton, people had noticed that rainbows existed and that glass could create colored patterns, but nobody had systematically investigated why. In the thirteenth century, Roger Bacon theorized that rainbows resulted from a process similar to light passing through glass or crystal. That was a reasonable guess, but it took four more centuries before anyone tested it rigorously.

Newton's famous experiment was elegant in its simplicity. He directed a narrow beam of sunlight through a glass prism and watched as it fanned out into bands of color—red, orange, yellow, green, blue, and violet (he added indigo later, for mystical reasons). Then he did something clever: he recombined those colored bands through a second prism, producing white light again.

This proved that white light wasn't pure; it was a mixture. The prism wasn't adding color to the light; it was revealing colors that were already there.

Newton hypothesized that light consisted of tiny particles—he called them "corpuscles"—with different colors moving at different speeds through transparent materials. Red corpuscles traveled faster through glass than violet ones, which is why they bent less sharply. This explanation was wrong about the nature of light (it's a wave, not a particle, though quantum mechanics would later complicate that picture), but it correctly described the phenomenon of refraction.

He was the first to use the word "spectrum" in this context. The Latin term originally meant "appearance" or "apparition"—something ghostly and insubstantial. Newton saw the rainbow-like display emerging from his prism as a kind of phantom revealing light's hidden composition.

The Colors That Don't Exist

Here's something counterintuitive: the spectrum doesn't contain all the colors you can see.

Pink isn't in the rainbow. Neither is magenta, brown, or white. These colors can only be produced by mixing multiple wavelengths together. When you see pink, your eye is receiving both red and blue light simultaneously—there's no single wavelength that produces that perception.

Colors that correspond to a single wavelength are called "spectral colors" or "pure colors." Everything else is a construction of your visual system, interpreting combinations of wavelengths as unified color experiences. Your brain doesn't just passively receive color; it actively synthesizes it from whatever mixture of wavelengths arrives at your retina.

This means the rainbow, while beautiful, is incomplete. It shows you only the colors that can be made from one wavelength at a time. The full range of human color perception extends into territories the spectrum cannot reach.

Why the Sky Is Blue (and Newton's Blue Wasn't)

The midday sky appears blue because of a phenomenon called Rayleigh scattering. Air molecules scatter shorter wavelengths more efficiently than longer ones, so blue light bounces around the atmosphere and reaches your eyes from all directions. Red and yellow light, with their longer wavelengths, pass more directly through, which is why the sun appears yellowish-white when overhead and reddish at sunrise and sunset, when its light travels through more atmosphere.

Interestingly, when Newton described the colors of the spectrum, his "blue" corresponded to what we now call cyan—a greenish-blue—while his "indigo" was closer to what we'd simply call blue today. The color names have drifted over three centuries. Isaac Asimov, among others, argued that indigo should be dropped from the standard rainbow colors entirely, since many people with normal vision can't distinguish it from blue or violet anyway. The human eye is relatively insensitive to indigo's frequencies.

But Newton's seven colors persist in cultural memory, a testament to the staying power of mystical numerology over empirical precision.

Beyond Newton: Discovering Invisible Light

Newton established that visible light could be separated into its component colors. But in the early nineteenth century, scientists began to suspect that the spectrum extended beyond what eyes could see.

William Herschel, the astronomer who discovered Uranus, made an elegant observation in 1800. He placed thermometers in different parts of a spectrum produced by a prism and noticed that the temperature rose as he moved from violet toward red. Then he placed a thermometer just beyond the red end of the visible spectrum—in what appeared to be empty space—and found the temperature rose even higher.

He had discovered infrared radiation: light too red for human eyes to see, but still carrying energy that could be measured as heat.

The following year, Johann Wilhelm Ritter discovered ultraviolet radiation by noticing that silver chloride, which darkens when exposed to light, reacted even more strongly to invisible radiation beyond the violet end of the spectrum. The visible spectrum was suddenly revealed as a small island in a vast ocean of invisible electromagnetic radiation.

Thomas Young, working around the same time, became the first to measure the actual wavelengths of different colors. He also proposed, along with Hermann von Helmholtz later in the century, that color vision depends on three types of receptors in the eye. This trichromatic theory turned out to be correct: humans see color because we have three types of cone cells, each sensitive to different (but overlapping) ranges of wavelengths.

The Machinery of Seeing

Your eye isn't a simple camera. It's a complex biological instrument with multiple layers of filtering and processing.

Light first passes through the cornea, the clear outer layer, which filters out most ultraviolet B radiation (wavelengths below 315 nanometers). Then it travels through the lens, which blocks most ultraviolet A radiation (315 to 400 nanometers). The lens also yellows with age, progressively filtering out more blue light over a lifetime. This is why older people sometimes see the world with a slightly yellowish tint—a condition called xanthopsia—and why their perception of the blue end of the spectrum may be slightly truncated.

People who have had their lenses removed (a condition called aphakia) can sometimes see into the ultraviolet range, because there's no longer a biological filter blocking that light from reaching their retinas. They may even experience cyanopsia—an abnormal bluish tint to their vision—because UV light can now excite their visual receptors.

The receptors themselves come in two main types. Rod cells handle low-light vision (scotopic vision) and don't distinguish colors. Cone cells handle daylight vision (photopic vision) and come in three varieties, each with different sensitivity curves. The combination of signals from all three cone types gets interpreted by your brain as specific colors.

The limits of what you can see depend on where these receptor sensitivity curves fall off. The long-wave (red) limit of your vision is determined primarily by your L-cones, which are most sensitive to longer wavelengths. Some people have genetic variations that shift their L-cone sensitivity—a condition called protanomaly—which changes where their red perception cuts off.

The Hidden Rainbow: How Other Animals See

The visible spectrum is defined by human vision, but humans are far from the most sophisticated seers in the animal kingdom.

Most mammals are dichromats—they have only two types of cone cells, not three. Dogs, for example, have cone cells peaking at 429 and 555 nanometers. They see most of the colors humans see, just with less differentiation between them. It's not that dogs see in black and white; they see a less colorful version of the world, similar to a human with red-green color blindness.

Horses have an even more truncated color vision, with cones at 428 and 539 nanometers, giving them reduced sensitivity to red light.

Birds, by contrast, are often tetrachromats—they have four types of cone cells, including one sensitive to ultraviolet light that humans can't see. Some birds can detect wavelengths as short as 355 nanometers. This isn't just academic: many birds have plumage patterns that are invisible to human eyes but vivid in ultraviolet, allowing them to identify mates and rivals through color signals we literally cannot perceive.

Fish are similarly well-equipped. Teleosts—the group that includes most familiar fish species—are generally tetrachromatic. Some fish using alternative light-absorbing molecules can extend their vision to 625 nanometers on the red end. The popular belief that goldfish can see infrared light is, however, a myth.

Insects offer another perspective entirely. Bees can see ultraviolet light, which helps them locate nectar in flowers—many flowers have ultraviolet patterns that serve as landing guides for pollinators. But bees can't see as far into the red as humans; their long-wave limit is around 590 nanometers. A red flower is essentially invisible to a bee.

Then there's the mantis shrimp, which has up to fourteen different types of photoreceptors, enabling a visible range from less than 300 nanometers (deep ultraviolet) to above 700 nanometers (near infrared). What it's like to perceive the world with that kind of visual bandwidth is essentially unimaginable.

Heat Vision: A Different Kind of Seeing

Some snakes have evolved an entirely different approach to perceiving electromagnetic radiation. Pit vipers, pythons, and some boas can detect thermal infrared radiation—wavelengths between 5,000 and 30,000 nanometers—using specialized pit organs on their faces.

This isn't visible light at all. It's heat radiation, the same kind of infrared that William Herschel detected with his thermometers. But these snakes can form actual images from it, perceiving warm-blooded prey as bright objects against a cooler background. A blind rattlesnake can strike accurately at a mouse by sensing its body heat alone. Some species can detect a warm body from a meter away.

This represents a fundamentally different sensory modality—not an extension of the visible spectrum but an entirely parallel way of imaging the world.

Reading the Light of Stars

The visible spectrum isn't just biologically interesting; it's scientifically revolutionary. Spectroscopy—the study of light broken down by wavelength—has transformed our understanding of the universe.

Every chemical element, when heated, emits light at specific wavelengths. These emission lines act like fingerprints. By spreading out the light from a distant star into its component wavelengths, astronomers can identify what elements that star contains without ever visiting it.

Helium was discovered this way. In 1868, astronomers observed an unfamiliar spectral line in light from the sun during an eclipse. They named the unknown element "helium" after Helios, the Greek sun god. It wasn't found on Earth until twenty-seven years later.

Spectroscopy also reveals motion. When a star or galaxy moves toward us, its light shifts toward shorter wavelengths (blueshift). When it moves away, the light shifts toward longer wavelengths (redshift). By measuring these shifts, astronomers can determine how fast distant objects are moving relative to Earth. This technique provided the first evidence that the universe is expanding—galaxies in every direction show redshifts proportional to their distance from us.

All of this flows from Newton's simple observation that white light passing through a prism breaks into colors. The spectrum became a tool for reading the composition and motion of objects billions of light-years away.

The Limits of the Window

Human visible perception evolved within constraints: the atmosphere filters certain wavelengths, the eye's components filter others, and our photoreceptors have limited sensitivity curves. The result is a narrow band of perception in a vast electromagnetic continuum.

But those limits aren't fixed. With age, the lens yellows, and blue perception diminishes. With certain surgeries, UV sensitivity can emerge. With genetic variation, the red limit shifts. With technology—infrared cameras, ultraviolet photography, radio telescopes—we can perceive radiation far beyond our biological limits.

The visible spectrum is both a window and a wall. It shows us a world of color, but it also hides most of what electromagnetic radiation could reveal. Every mantis shrimp, every pit viper, every bee that pollinates a flower by patterns we cannot see reminds us that our perception is not the world itself—it's just one way of sampling it.

Newton gave us the word "spectrum" to describe what his prism revealed. But the apparition he named is far stranger and more limited than he knew. The rainbow is not a full accounting of light. It's a glimpse through a narrow window, shaped by the peculiarities of evolution and the accidents of atmospheric physics.

We see what we evolved to see. And what we evolved to see is beautiful—but it's only a fraction of what's there.