From Infrared to X-rays, the Curious History of Invisible Light


By Bob Berman

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How much do you know about the radiation all around you?

Your electronic devices swarm with it; the sun bathes you in it. It’s zooming at you from cell towers, microwave ovens, CT scans, mammogram machines, nuclear power plants, deep space, even the walls of your basement. You cannot see, hear, smell or feel it, but there is never a single second when it is not flying through your body. Too much of it will kill you, but without it you wouldn’t live a year.

From beloved popular science writer Bob Berman, Zapped tells the story of all the light we cannot see, tracing infrared, microwaves, ultraviolet, X-rays, gamma rays, radio waves and other forms of radiation from their historic, world-altering discoveries in the 19th century to their central role in our modern way of life, setting the record straight on health costs (and benefits) and exploring the consequences of our newest technologies.

Lively, informative, and packed with fun facts and “eureka moments,” Zapped will delight anyone interested in gaining a deeper understanding of our world.


It is obvious that we must attach a deeper and much wider meaning to the word light than has hitherto been ordinarily understood.



It’s everywhere.

At this moment, as you sit quietly reading this book, you are awash in it. At work, it’s emanating from your electronic devices; step outside for lunch, and the sun bathes you in it. You may receive an extra dose of it when you visit your doctor, pass through security at the airport, or drive through city streets, but minuscule amounts of it are with you always. You cannot see, hear, smell, or feel it, but there is never a single second when it is not flying through your body. Too much of it will kill you, but without it you wouldn’t live a year.

“Invisible light” seems like a contradiction. Like Simon and Garfunkel’s “The Sound of Silence,” it’s an oxymoron. We think of light, by definition, as something seen, something that enables our seeing, illuminating the darkness. Unlike dogs, who sniff in order to “get” what an object is, we depend on vision above all our other senses. We rely on light to tell us about our surroundings.

But just as there are frequencies of sound audible to other animals that we cannot hear, there is a whole world of light outside our range of vision, a world that is humming with activity. Though we rarely think about this invisible world, our way of life depends on it. It’s because of invisible light that you can do things such as send a text message, use GPS to find your way to a friend’s house, listen to the radio, or microwave a frozen pizza. Invisible light shows us things we would never otherwise see, including our own skeletons and brains and the history of our universe.

I was reminded of just how much we rely on invisible light, and how mysterious it remains to us, during a visit from my sister and her family. It was a lazy summer afternoon, and we were sprawled across a few couches sharing a bowl of popcorn. My niece, her shoulders crimson after a day outdoors, was chatting on her cell phone, holding up a promising finger to her mother, who was scolding her for not using sunscreen. My brother-in-law, meanwhile, was asking my opinion on an article he’d read proposing that Wi-Fi be banned in schools because of its dangers. We were all depending on invisible light (for the microwave popcorn, the cell-phone service) while being concerned that it might harm us (sunburn, mysterious Wi-Fi health threats) and confused about what to do to protect ourselves.

We need it, and it surrounds us, yet we remain uneasy about living with invisible light, partly because we fear what is unknown. After all, most of us don’t know much about “all the light we cannot see.” This book aims to change that.

My hope is to expose the hidden side of the spectrum, to make the invisible (at least temporarily) visible and vivid to you. As you’ll see, each of the different varieties of invisible light — from gamma rays to infrared to ultraviolet — has its own characteristics and peculiarities, as distinct as red is from blue. We’ll meet rays that pass through solid matter instead of being reflected by it and others that cause water to boil. Some come from deep space and zoom through astronauts’ brains; others are left over from the birth of the universe. You may be surprised to find how much of our world and our history is touched by invisible rays. They rescued lives when the Titanic went down. They help determine the daily weather. Some can produce sudden, lethally destructive damage in human bodies.

We’ll split our exploration into two equally important parts. In some chapters we’ll go back in time to meet the pioneering scientists who first “saw” the invisible. Until the eighteenth century, no one had the faintest idea that there might be such a thing as light that cannot be detected by human vision, and until the nineteenth century no one had any proof of its existence. But once the discoveries of invisible light began, they avalanched, until most of today’s indispensable aspects of life, the ones we take for granted, depend on them.

In other chapters we’ll explore how these phantoms affect our lives and our bodies as they provide their near-magical conveniences, from the cell phone in your pocket to the radio in your car. What role does invisible light play in our medicine, our technology, and our culture in the twenty-first century? What new opportunities for its use are on the horizon?

Like my family lazing around the living room that summer afternoon, you probably have questions about how invisible light is affecting your health. What are the microwaves from your cell phone doing to your brain? What is radiation, and how much of it are you exposed to? Which invisible ray causes the most annual deaths? Which foods are most radioactive? This book will answer all those questions, clarifying once and for all the controversial claims about radiation’s health consequences. Some of the things you’ll learn will soothe you (ultraviolet light can decrease your risk of cancer), and some will shock you (a single whole-body CT scan delivers more radiation than was received by Hiroshima survivors a mile from ground zero), but in every case context is key. Myths will be busted, and wild facts will abound.


Light Fantastic

If God really did say, “Let there be light,” it was no small housewarming present. There is a lot of light in the universe — one billion photons of light for every subatomic particle. In terms of individual items in the cosmos, including the components of atoms, photons constitute 99.9999999 percent of everything. The universe is literally made of light. And that includes not only ordinary everyday visible light but also the vast majority of light — the kind we cannot see.

Light is an astonishing entity, and the quest to understand it has obsessed the greatest thinkers in disparate cultures through the centuries. The ancient Greeks, probably by sheer dumb luck, were the first to hit upon a key aspect of visible light — that it does not exist independent of us as observers. Physics now tells us that light is made up of intertwined magnetic and electrical fields. Since neither magnetism nor electricity is visible to our eyes, light is inherently invisible.

When we look at a bright orange sunset, we’re not directly perceiving actual light. Rather, the energy reaching us from those electromagnetic pulses stimulates billions of neurons in our retinas and brains, which then fire to arouse a complex neurological architecture that produces in us the sensation of orange. An entire biological empire is thus as essential to the existence of brightness and colors as the photons themselves.

The Greeks didn’t know anything about brain structure, of course, yet they still figured out that light is a sensation, with no existence independent of the observer — which was either amazingly perceptive or just a lucky guess. But the Greeks had light’s direction wrong. Knowing that its speed appeared instantaneous, they didn’t imagine that a pulse of light originating in a candle sped in our direction until it struck our eyes. On the contrary, they regarded light as a ray traveling outward from our pupils. This belief, that our eyes project an illuminating beam, was universally embraced for more than a millennium. Even so, a few early iconoclasts envisioned eyesight as an interplay between this supposed eye ray and something emitted by other sources.

The classical thinker who came closest to the truth about light was the Roman Lucretius, who in the first century BCE, in his On the Nature of Things, wrote, “The light and heat of the sun are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air.”

Lucretius’s view of light as particles — later supported by Isaac Newton — included that profound “lose no time” characterization, showing that he believed light moved immeasurably fast. But whether scientists considered its speed merely super-quick or instantaneous, light remained popularly regarded as a phenomenon that originates in the eye for centuries to come.

The first true breakthrough came from the mathematician and astronomer Alhazen — formally known as Abu Ali al-Hasan ibn al-Hasan ibn al-Haytham — who lived in Egypt during the golden age of Arab science. Around the year 1020, when the rest of the world was in the intellectual coma of the Dark Ages, Alhazen said that vision results solely from light entering the eye; nothing emanates from the eye itself. His popular pinhole camera obscura, which drew astonished yelps of wonder when visitors observed the phenomenon, gave weight to his arguments, for here was a full-color “motion picture” of nature splayed out on the walls. But Alhazen went much further. Light, he said, consists of streams of tiny, straight-moving particles that come from the sun and are then reflected by various objects. Sounds simple, perhaps, but Alhazen’s spot-on conclusions were six centuries ahead of anyone else’s.

The Renaissance turned up the juice on the “What is light?” debate, which eventually took on the quality of a food fight. In the late seventeenth century, Newton joined astronomer Johannes Kepler in arguing that light is a stream of particles, while men such as Robert Hooke, Christiaan Huygens, and, soon, Leonhard Euler insisted that light is a wave. But what is it a wave of? They thought there had to be a substance doing the waving, so these Renaissance scientists decided that space was filled with a plenum (later called an ether), an invisible substance that facilitated the movement of magnetic and electrical energy.

One obvious fact managed to sway many in favor of Newton’s particle idea. When light from the sun passes a sharp edge, such as the wall of a house, it casts a sharp-edged shadow on nearby objects. That’s what particles moving in a straight line should do. If instead light were made of waves, it ought to spread out — diffract — as ocean waves do when passing a jetty. To the particle proponents, the existence of sharp-edged shadows, combined with Newton’s reputation as a genius, made the wave proponents seem like nut jobs.

The particles-versus-waves controversy eventually took a curious turn. As if some wise King Solomon ruled nature, everyone was soon declared right. The biggest breakthrough came from Scottish physicist and mathematician James Clerk Maxwell, who in 1865 showed that all light is a self-sustaining wave of magnetism with an electric pulse wiggling at right angles to it. One type of pulse stimulates the other, so that both the electrical and the magnetic waves continue indefinitely. From then on, science called light an electromagnetic phenomenon.

All light consists of a dual wave. A magnetic pulse is accompanied by an electric pulse positioned at a ninety-degree angle to it. (Molecular Expressions at Florida State University)

But where did this phenomenon come from? In 1896, the Dutch physicist Hendrik Lorentz figured out that the existence of a strange phenomenon — light splitting itself in two within a strong magnetic field — must mean that the rapid motion of some tiny, unknown, negatively charged particle has to be the source of all light everywhere in the universe. A year after he drew this astoundingly prescient conclusion, the first subatomic particle was discovered. This was the electron, whose movements are indeed the sole creator of all light. For predicting the existence of the electron before its actual discovery, Lorentz won the Nobel Prize in 1902.

How exactly is light — any light, all light — created? When an atom gets struck by energy — from a quick zap of electricity or a collision with a stray electron or the introduction of heat — its wiggling motion is jolted into a greater speed. This extra energy excites the atom’s electrons, which give a figurative yelp and jump to an orbit farther from the nucleus. They don’t like to be there, so in a fraction of a second they fall back into a closer, smaller orbit. As they do so, the atom surrenders a bit of energy. Since energy is never lost under any circumstances, this energy must manifest itself in some other way. And it does. A bit of light, a photon, materializes out of the emptiness as if by magic — then instantly rushes away at its famous breakneck speed. That’s the only way light is ever born. Out of seeming nothingness, whenever an electron moves closer to its atom’s center. Simple, really.

So light can be thought of as a set of two waves, one of electricity and one of magnetism, or as a weightless particle called a photon. Taking our cue from Albert Einstein, we might visualize a photon as a tiny bullet, an energetic speck with no mass, weighing nothing and with the curious property of being unable to ever stop moving. Nowadays, most people who think about such things (we science nerds) find it easiest to visualize light as a wave when it’s en route from point A to point B and as a photon when it finishes its journey by crashing into something. But one may call light a photon or a wave and be equally correct.

The twentieth century brought us quantum theory, which — in addition to showing that solid objects such as electrons can behave as energy waves, too — revealed something extremely weird: when an observer uses an experimental apparatus to determine the location of photons or subatomic particles such as electrons, these entities always behave as particles and do things only particles can do, such as pass through one little hole or another but not both at once. But when no one’s measuring where exactly each photon is situated, they behave as waves that blurrily pass through both holes in a barrier simultaneously to create an interference pattern on a detector located beyond the openings — which only waves can do.

Thus the observer and, weirdly, the information in his or her mind plays a critical role in whether light exists as a wave or as a tiny discrete object. The same is true for particles of matter. What you see depends on how you observe and what you know. Most physicists now think that a human consciousness is required to make a photon or an electron’s “wave function” collapse so that it occupies a particular place as a particle. Otherwise it’s just a theoretical object with neither location nor motion.

Just a century ago, the local realism mind-set of science, and even common sense, held that all objects, including atoms and photons, have an existence independent of our observation of them. But that’s been replaced by a more modern view — that our observation itself is necessary for the very existence of photons and electrons, a spooky prospect.

But does an electron’s wave function collapse and turn into an actual particle if a cat is watching? Would light always be waves and never discrete photons if no humans were around? Our best answers are “Who knows?” and “Yes” respectively, but obviously this whole business is Wonderland-strange.*

Let’s make this strangeness clearer. A century ago, if we detected a bit of light (or even a physical particle) arriving at an instrument with which we could measure its incoming direction, we’d have confidently plotted out its previous path. No longer. Now we say that it had no path before we started to observe it. It possessed no real existence as an actual photon or electron or whatever it was. Rather, its observed existence is its only existence. Observation establishes reality. Nothing else is certain. As the late physicist John Wheeler put it, “No phenomenon is a real phenomenon until it’s an observed phenomenon.”

Which brings us to our next question: why can we observe some kinds of light with the naked eye and not others?

* My friend Matt Francis, an electron microscopist, is training his dog to recognize and respond to light displayed as a wave pattern on a screen as opposed to a series of particles. If he succeeds in teaching the dog to bark when observing waves and remain silent when observing only particles, he may be able to settle the matter and determine whether a dog’s consciousness can “collapse” a photon into its particle configuration. Yes, such issues actually obsess some of us.


Now You See It, Now You Don’t

All forms of light, from the visible to the invisible, reside on the electromagnetic spectrum. Along this range there are many kinds (and colors) of light, and each variety can be distinguished from the others by two straightforward properties.

The first is wavelength. The length of each individual light wave varies from the tiniest fraction of an inch to more than a mile and spans everything in between.

The second is frequency, meaning the period of time it takes the wave to pass you and be replaced by the next wave, as if you were seated in a reviewing stand watching the light parade before your eyes.

Think of an ocean wave. In the open sea, a typical wave is around one hundred yards, or ninety-one meters, long — roughly the length of a football field. Its frequency is a bit less than one second. This means that each wave’s peak requires nearly a second to pass any given point and be replaced by a trough, which in turn is followed by the peak of the next wave.

Science can identify any wave, or any particular type of light, by either its length or its frequency. For example, each wave of green light at a traffic signal has a length of 530 nm (or nanometers, meaning 530 billionths of a meter), which is about one millionth of an inch. These tiny waves have a frequency of 530 THz, or terahertz, which means that 530 trillion of them pass your eye each second. (That the number 530 appears in both wavelength and frequency is a coincidence; the matchup is true of green light but not of any other color).

When the signal turns red, you perceive waves of a longer length — twice as long, in fact. Each crimson-light wave is around two millionths of an inch from crest to crest. Red light has the longest waves of all visible light, but they’re still smaller than most germs in our body. These waves vibrate more slowly than green light, too, with “just” 450 trillion wave pulses occurring per second. What’s important is that all the light we see has wavelengths somewhere between 400 and 700 nm, which used to be expressed as 4, 000–7,000 angstroms. The light we cannot see has wavelengths either shorter or longer than that.

Short waves pulse, or change, more quickly than long ones, and this gives them more power, or energy. As a result, while the light we can see is too weak to break atoms apart, fast-vibrating light such as ultraviolet light can indeed strip an atom of one or more of its electrons, which alters molecules and can lead to consequences such as carcinogenesis.

Invisible light has generally been named according to either its wave size or its position on the spectrum compared to the visible colors. Thus infrared light occupies a place just before the visible red light in the spectrum, meaning its waves are a bit longer than the red-light waves coming at you when you’re stopped at a traffic light. By contrast, ultraviolet light lies just after violet light, and its waves are slightly shorter.

The weakest kind of light is a radio wave. The longest radio waves measure a thousand miles from crest to crest. By contrast, the distance from one visible light wave to the next is just one millionth of a meter, or one hundred-thousandth of an inch. A few hundred trillion visible waves pass you every second. Even more mind-bogglingly short and fast are gamma rays, the strongest kind of light, with crests spaced just a trillionth of a meter apart and frequencies of a billion trillion per second. All other parts of the spectrum lie in between radio waves and gamma rays.

Visible light occupies only a tiny part of the electromagnetic spectrum. (Wikimedia Commons)

Except for the dim glow of the stars, all light is ultimately solar. Moonlight is reflected sunlight. The aurora borealis results from solar particles electrically stimulating the sparse oxygen atoms a hundred miles up. Candlelight and other kinds of flame require combustible materials such as coal, wood, and oil, which are forms of stored energy from long-dead plants and animals that would never have existed without the sun.

In our era we also create light using electricity, but that, too, comes from burning oil, gas, coal, or hydropower generated by falling water, which would never circulate back to higher altitudes without everyday solar warmth. Only nuclear power and starlight are independent of the sun, and stars emit exactly the same visible and invisible rays as our own sun does. Stars differ only in their proportions: hot, massive stars emit copious ultraviolet rays and blue light, whereas the more numerous lightweight stars give off copious reds, oranges, and infrared radiation, with very few UV rays. Rather poetically, our eyes see only the colors the sun emits most strongly. Our retinas are designed to perceive sunlight’s most abundant energies and nothing else. So we really do have a sun bias. In a way, we scan the universe through the sun’s eyes.

As we learned in science classes at school, the sun’s white light is merely our retinal and neurological response to receiving all the sun’s component spectral emissions at the same moment. White means we’re getting it all. In a very real sense, white is a rainbow in a blender.

Indeed, if a scientist looks through a spectroscope, which reveals the true colors in the object she is studying by “unscrambling” them, a cloud that appears white to the unaided eye will resemble a vivid rainbow. The instrument reveals that a white cloud is actually composed of red, orange, yellow, green, blue, indigo, and violet light, and when those colors hit our eyes all at once, we see white. Studies conducted way back in the eighteenth century showed that even if a few of those colors are absent — orange and violet, say — we will still see white. Turns out all that’s needed to make white are blue, red, and green combined in equal measure. These are called light’s primary colors (totally different from yellow, cyan, and magenta, the primary colors of paint and pigment). So if we see white, it means we’re receiving red, blue, and green light simultaneously.

If they’re combined unequally, those same three primary colors will create others. Your computer and your TV use this trick all the time. If a friend sends you a digital photo of autumn foliage, you might see that some leaves appear deep reddish-purple on the screen. Your computer creates this effect by mixing, say, eighteen parts blue, seven parts red, and one part green. Just three hues, commonly called RGB, for red, green, and blue, can combine to create every possible color.

Some combinations are not logically obvious. Guess what mixture is needed to create yellow light? The answer is an equal blend of green and red. This surprises many people, because it seems logical that mixing red and green light would create a sort of reddish-green sensation. But no. Yellow is our eyes’ reaction to red mixed equally with green.

On the other hand, if we mix red and green paint instead of red and green light, we won’t get yellow but rather some muddy brownish-black hue. That’s because entirely different rules apply to paint. Pigment does not glow on its own; we see a paint color only because some external white light (e.g., the ceiling lamp or the daylight streaming in from windows), is hitting the palette or painting, where the pigments absorb some of the white light’s colors and reflect others. Yellow paint, for example, looks yellow because its chemical absorbs white light’s blue component but reflects the light’s red and green components — and the combination of red and green light always yields the sensation of yellow.

We see pigments and paints by a subtractive process. If you’ve ever painted, you may have had the frustrating experience of trying to create a color by mixing others — and ending up with a palette covered in brown. That’s because each new color you add introduces further subtractions from the white light illuminating the room, reducing reflections from the canvas so that less light reaches your eyes. Further pigment combinations invariably darken the image. Add too many pigments, and the result is muddy brown or black, because by that point nearly all light is absorbed by the paint’s molecules and nothing is reflected to our eyes. But light is a different story. Adding more light always brightens an image.

The experience of vision is a symbiotic event. This is so fundamentally important yet so little known that it bears repeating from the previous chapter: by itself, light has no color or brightness. Light is merely a wave of magnetism and, at right angles to it, a wave of electricity. So the real external world is as utterly invisible as radio waves. Without humans to perceive it, “external reality” is nothing but a complex jumble of various blank energy frequencies. But when stimulated by these invisible frequencies, our six million cone-shaped, photon-sensing retinal cells respond, each to a rather narrow set of predetermined vibrations. Thus stimulated, they send an electrical signal at 250 miles per hour up the optic nerve until several hundred billion neurons in the rear of the brain fire in a continuous, complex way. The result is an image perceived in the brain as a color, such as blue.

Bottom line: the “external world” is an internal experience. On their own, colors are not “out there.” When no one is looking, a sunset has neither color nor brightness. It is an invisible mélange of electrical and magnetic pulses.

Some of us undergo an unusual subjective experience when confronted with the sun’s photons. Deuteranopes — the 10 percent of males who lack the green retinal receptor — see far fewer colors than the rest of us do. To them, shades of red and green can look identical — to each other as well as to what we perceive as yellow, the combination of the two. Theirs is a world of blues and yellows. They don’t understand why the rest of us are so enchanted by a rainbow, because to them it’s merely a swath of two colors. These people can easily run traffic lights if the bulbs are in unfamiliar positions. Turns out dogs and elephants are deuteranopes, too. It’s one of the reasons we should never let them drive.


  • "An enthusiastic account of all the light we cannot see from a science popularizer with a knack for presenting hard facts clearly and stylishly. . . . A guide for laymen written with gusto and assurance."—Kirkus
  • "The narrative is briskly conversational: We're on the porch, shooting the breeze with a knowledgeable neighbor. Mr. Berman's avowed goal in writing this book, he says, was 'to open a window onto the enormous universe of omnipresent energies.' Once that window is thrown open, it is hard to look at the world the same way." Wall Street Journal
  • "Nimbly busts common myths ... Erudite but never stuffy, Berman writes with enthusiasm and clarity, making this an informative and digestible read for the science-curious."—Booklist
  • "[Berman's] an unfailingly congenial explainer, always ready with the kinds of fascinating facts his readers might have missed in school."—Christian Science Monitor
  • "Captivating ... fear not the long-winded scientific discourse: Berman zings through historical and scientific adventures."—American Scholar
  • "Astronomy writer Berman runs through a fascinating history of the rainbow's invisible bands in this breezy, accessible read... In the style of a favorite professor, Berman injects bits of odd humor and captivating tangents into this complex but familiar topic."—Publishers Weekly
  • "[Berman] excels at making complex concepts accessible for lay readers...this is a great option for those curious about history, theories, and function of everyday things."—Library Journal
  • "Explaining light using anecdotal history and colloquial explication, Zapped makes entertaining sense out of what could be dry math and physics."Shelf Awareness

    "Entertainingly kinetic.... [Berman] transmits science geekery in vivid prose stuffed with unexpected insights and arresting observations.... Absorbing."
    --- Michael Benson, New York Times
  • "Vastly entertaining.... Zoom is invaluable for everyone who once knew Newton's three laws and would like a refresher, but it is more fun than that.... Bob Berman knows how to make science accessible."
    --- Kate Tuttle, Boston Globe
  • "An engagingly quirky popular treatment of the ongoing debate about the nature of space and time in the universe and our place as both observers and participants."
    --- Kirkus Reviews

On Sale
Aug 8, 2017
Page Count
272 pages

Bob Berman

About the Author

Bob Berman, one of America’s top astronomy writers, is the author of Zapped, Zoom, and The Sun’s Heartbeat. He contributed the popular “Night Watchman” column for Discover for seventeen years and is currently a columnist for Astronomy, a host on Northeast Public Radio, and the science editor of The Old Farmer’s Almanac. He lives in Willow, New York.

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