Electromagnetic Spectrum
One Field, Every Frequency
One Ripple, Every Size
Radio, light and X-rays are one ripple in one field. A charge shaken up and down sends it out, and it runs away at the speed of light, about 300,000 kilometers a second. Far from the charge, its electric and magnetic parts rise and fall together, in step. What differs from one kind to another is how often it repeats, its frequency, and so how far apart its crests are, its wavelength. The speed is the same for all, so the two go together: in empty space, the frequency times the wavelength is always the speed of light. Shaken a hundred million times a second, as in an FM radio transmitter, a charge sends out crests 3 meters apart. Shaken six hundred trillion times a second, it sends crests half a thousandth of a millimeter apart: green light. The strength of the wave follows how sharply the charge is shaken to and fro, so the faster it is shaken, the smaller the swing that sends as strong a wave.
The field’s equations say that such a ripple runs at the speed of light, and that is how light was recognized as one. Nothing in them limits how often it may repeat, and each band beyond our sight was found by a different trick. A thermometer moved past the red end of a prism’s colors warms: infrared. Silver chloride darkens past the violet end: ultraviolet. Sparks make waves far too long to see. X-rays and gamma rays were found last.
The span is enormous. Lightning keeps the gap between the ground and the upper air ringing with radio waves longer than Earth is wide, about eight crests a second. The most energetic gamma rays yet caught came from a pair of stars in the constellation Cygnus, where a collapsed star, a black hole or a neutron star, pulls gas off a massive companion and fires out jets. Those gamma rays’ waves were tens of millions of times shorter than an atomic nucleus is wide. From one end to the other the wavelength changes by a factor of 10²⁹. Our eyes see less than one doubling of frequency out of nearly a hundred.
Shorter Waves, Bigger Lumps
The field’s ripples take and give energy only in lumps, photons, and the size of a lump is its frequency times Planck’s constant. So the shorter the wave, the bigger the lump. Physicists measure lumps in electronvolts, the energy an electron gains crossing one volt. A photon of green light carries about 2.5 electronvolts. One from a Wi-Fi router carries about a hundred-thousandth of one. A photon from a dental X-ray carries tens of thousands, and a gamma ray from radioactive cobalt more than a million.
Everything at room temperature jiggles, and the energy of that jiggle is about a fortieth of an electronvolt. A radio lump is thousands of times smaller than that jiggle, so no ordinary detector can catch one alone. An antenna feels countless lumps at once, as a smooth push and pull of the field. A gamma-ray lump is millions of times bigger than the jiggle, and each one is caught alone, whole and in one spot: a click in a Geiger counter. It is the same ripple at both ends, always traveling as a wave and always caught in whole lumps. With radio the lumps are too small to notice one by one; with gamma rays each one stands out.
Matter Answers in Steps
A lone atom or molecule can take a lump of light only if the lump fits a step it can make, or is big enough to tear an electron away or break a bond. An atom’s electrons can only change from one set pattern to another. A molecule has smaller steps besides: it can turn, and its bonds can stretch and bend. Each kind of motion has its own size of step, and each band of the spectrum is the size of one of them. The smallest steps of all, flipping the tiny magnet a proton carries, as a hospital’s magnetic scanner does, take radio. Flipping an electron’s stronger magnet takes radio or microwaves. Turning a molecule takes microwaves. Stretching or bending a bond takes tenths of an electronvolt, infrared. Moving an outer electron to a higher pattern takes a few, visible light and ultraviolet. Moving an electron deep inside a heavy atom takes thousands, X-rays, and a step of the nucleus itself usually takes hundreds of thousands to millions, gamma rays. Every kind of atom and molecule has its own set of steps, so a thin gas of it takes and gives light only at its own frequencies, its lines, and the lines in a star’s light tell what the star is made of. In a liquid or a solid, crowded neighbors blur the steps into broad bands, so these take whole ranges of lumps.
So water is clear. Dragging its molecules round and shaking their bonds take microwaves and infrared, and its electrons take only ultraviolet. Only faint echoes of the shaking reach into the visible and take a little red, so a glass of water looks clear and a deep pool blue. Glass is clear for the same reason, and dark to the ultraviolet that burns and to infrared beyond a few micrometers. A molecule of carbon dioxide bends when it takes infrared with a wavelength of 15 micrometers, close to where Earth’s own warm glow is strongest. So it takes in part of that glow and, in collisions, passes the energy on to the air round it. The air’s carbon dioxide glows in turn, at the air’s own temperature, in every direction, some of it downward, and that slows the flow of heat from Earth to space.
A microwave oven does not hit a step of water. A lone molecule, in steam, turns in steps and takes only microwaves that fit them, such as those of 22 and 183 billion swings a second, by which satellites measure the water in the air. In a liquid, molecules cannot turn freely. The oven’s field drags them round against their neighbors, and the drag heats the food. That drag is strongest near 20 billion swings a second, but ovens use 2.45 billion, so the waves reach a centimeter or two in instead of all being taken in the outer millimeter.
A lump can also go in big and come out small. An ultraviolet lump moves an electron into a pattern of much higher energy. Some of that energy goes into shaking the molecule, and when the electron returns to its lowest pattern it gives out a smaller, visible lump. That is fluorescence: the glow of some minerals under an ultraviolet lamp, and of white shirts washed with brighteners.
One Lump at a Time
A molecule takes a lump whole or not at all, and two lumps almost never join forces in one molecule. So what light can do to a molecule is set by the size of its lumps, not by how bright it is. A lump of the ultraviolet that burns, about 4 electronvolts, can weld two neighboring letters of DNA together, and the cell must cut the damage out. That damage is what sets off a sunburn. Red light from the Sun or any lamp never does it, however bright. Its lumps are too small, and all they can do is warm.
Above about 10 electronvolts a lump can tear an electron right off the atoms and molecules of air and of living things. That is what ionizing radiation means: far ultraviolet, X-rays and gamma rays. Radio, microwaves and visible light are not ionizing: no single lump of them is big enough, and the fields of a lamp or a phone are far too weak for lumps to join forces. Only a focused, pulsed laser crowds them enough to tear electrons off, which is how such a laser makes a spark in air. The radio waves of a phone held to the head warm it, by a fraction of a degree at most.
Light that frees electrons from a metal gives them a top speed set by its color, not by its brightness: brighter light frees more of them, not faster ones. Lumps of light explain that neatly, though a smooth wave meeting a metal’s steps can explain it too. The plainest proof is newer. Make lumps in pairs, and let one of each pair signal that its partner is on the way. Send the partner at a half-silvered mirror, with a detector behind each way out. Each lump clicks one detector or the other, almost never both, where a smooth wave would split and could click both.
Through Walls, Through Flesh
What gets through a thing depends on its steps and on its size. Brick, wood and plaster have few free electrons and no steps that fit radio’s tiny lumps, so radio crosses walls, and only the water in damp concrete takes much of it. For light, a wall is a heap of grains and fibers, each larger than its waves. Each one bends light and throws it aside, pigments take the colors that fit their steps, and within a fraction of a millimeter all of it is turned back or taken. Radio waves, hundreds of thousands of times longer, pass the same grains as if they were not there.
A sheet of metal stops radio: its free electrons follow any slow push of the field and throw the wave back. Holes much smaller than a wave stop it too. The mesh in a microwave oven’s door has holes a millimeter or two across, and the oven’s waves are 12 centimeters long, so they bounce back. Light, whose waves are far smaller than the holes, passes, and you can watch the food.
X-ray lumps are far bigger than any step of the light atoms in skin and muscle, mostly hydrogen, carbon, oxygen and nitrogen, so most of them pass through a hand, and a small share through a whole body. Calcium in bone holds its inner electrons much more tightly, nearer the size of an X-ray lump, and takes far more of them. Within weeks of their discovery, doctors were using X-rays to find broken bones and bullets.
Gamma rays pass most things. A gamma-ray lump is far bigger than even the deepest step in an atom, so it mostly scatters off electrons, handing each part of its energy. Stopping gamma rays takes many electrons in the way, and so thick layers of dense matter: about a centimeter of lead halves the gamma rays of radioactive cobalt.
Everything Glows
Every warm thing shakes its charges at random, and so it glows, best at the frequencies it takes in best. A dense, dark thing takes in nearly every frequency, so it glows across a broad, smooth spread of them, brightest near one set by its temperature counted from absolute zero: twice as hot, twice the frequency of its brightest light. Far above that peak its glow dies away fast, because its random jiggle seldom gathers enough energy for a lump so big. At 37 degrees Celsius your glow is infrared, mostly at wavelengths from 5 to 30 micrometers, and a thermal camera sees you by it.
Heat a solid in a dark room, and near 525 degrees Celsius its glow first reaches the eye, as a dull red. Almost every solid starts to show at that same temperature. Hotter, it glows orange, then yellow, then white. The filament of an old-style light bulb, at about 2,500 degrees, glows yellow-white, and still only about a tenth of its light is visible. The rest is infrared, which is why such bulbs ran so hot, and why LEDs, which make light from electrons dropping across a gap in their crystal rather than from being hot, have replaced them.
The Sun’s surface is at about 5,500 degrees. Added up in equal steps of wavelength, its light is brightest in green. Added up in equal steps of frequency, it is brightest in the near infrared. The two counts differ because a step of wavelength at the violet end spans about three times as many frequencies as the same step at the red end. Either way the Sun holds every color at once, so it looks white, not green, and about half of its energy is infrared. Gas falling into black holes and filling clusters of galaxies, at millions of degrees, glows in X-rays. And a cold glow fills the whole sky: light from when universe was a glowing gas at about 3,000 degrees above absolute zero, stretched by the expansion of space into microwaves at 2.7 degrees above absolute zero.
Why We See This Band
Our eyes see from about 400 to 700 nanometers, and three reasons meet there. The first is sunlight, brightest here when counted by wavelength. Counted by frequency, its brightest part lies in the near infrared, so this alone does not decide it. The second is clear air and water. Air passes light from about 300 nanometers, where ozone starts to stop it, into the near infrared. Water, where eyes first evolved, is clear only in a narrow band centered on blue, and a few centimeters of it take the infrared.
The third is chemistry. A visible lump, 1.8 to 3.1 electronvolts, is just enough to flip a molecule from one shape to another, as light flips the retinal in the eye’s rhodopsin, and not enough to break it. An infrared lump would be lost in the eye’s own warm jiggling. An ultraviolet one, bigger, would slowly wear out the molecules that catch it, and the lens of our eye stops most of it before it reaches the back of the eye.
Radio reaches the ground too, through a window of its own. But an eye makes a sharp picture only of waves far shorter than itself. To see as sharply as we do with waves a meter long, an eye would need to be a few kilometers across. Radio astronomers build such eyes from many dishes spread over the ground. Other animals see a little past our edges: bees see near ultraviolet, and pit vipers feel the infrared of warm prey through pits on their faces, not with their eyes.
The Sky in Every Band
Air stops most of the spectrum. The shorter ultraviolet, most of the infrared, and all X-rays and gamma rays are stopped high up, so telescopes for most of them fly in space. The most energetic gamma rays are caught from the ground, by the showers of particles and faint flashes of light they set off in the air. Out there too, what passes depends on size. Grains of dust between the stars, about a tenth of a micrometer across, are close enough in size to the waves of visible light to scatter them, but far smaller than infrared and radio waves, which pass. Dust dims the visible light from the center of our galaxy about a trillion times, and its near infrared only about tenfold.
And each band shows different things. Radio shows fast electrons spiraling round magnetic fields, and the hydrogen between the stars, which sends out waves 21 centimeters long when the spin of an atom’s electron flips from along its proton’s to against it. Infrared shows warm dust and, stretched by the expansion of space, the earliest galaxies yet seen. Visible light shows stars. X-rays show gas at millions of degrees, and gamma rays the most violent things of all: one gamma-ray telescope in orbit catches about 240 gamma-ray bursts a year, flashes from collapsing stars and colliding neutron stars.
It is one field at every frequency, and one set of equations. Along the spectrum only how often the ripple repeats changes, and with it the size of its waves and of its lumps, and so which motion of matter answers.
