People have been trained to be scared of the word “radiation”. But all it means is that something spreads outwards from a source. Sound counts as radiation. So do ripples on a pond, or earthquake waves in solid rock. If you use the term broadly enough, the shrapnel that blasts outward from a grenade can be considered as a form of radiation. And, of course, light is a form of radiation.
What people need to be legitimately scared of is the narrower category of ionizing radiation. That’s the nasty stuff that comes out of radioisotopes, nuclear reactions, and x-ray machines. It’s bad for ya because it destroys protein and DNA molecules inside your cells. Anything that’s capable of ionizing an atom is also capable of breaking apart an organic molecule in the process, and if that molecule is inside you, damaging enough of them in this way will give you radiation sickness, cancer, three-headed children, and so on.
The hot particles which spit out of radioisotopes, and which are generated in tremendous floods by nuclear reactors and bombs, are ionizing radiation. They aren’t waves, like sound or light (at least, no more than any solid object is wavelike) — they’re the subatomic equivalent of grenade shrapnel, consisting of solid pieces flung through the air. But the way that they shine straight out in all directions is such that the word “radiation” has never gone out of fashion for describing them.
X-rays and gamma rays are also ionizing radiation — especially the latter, which tend to be produced in nuclear reactions and accompany the other emissions of radioisotopes. But unlike the alpha and beta and other hot particles, these are electromagnetic waves. They are, essentially, light.
The heat on your skin from standing in front of a fire is also a form of light (infrared), but it is not ionizing. The only way it’ll ever disrupt organic molecules is by cooking them, and it’s far less effective for that purpose than, say, contact with hot water. But how can light be both ionizing and non-ionizing?
It comes down to the wave-particle duality. I’ve spoken of subatomic particles and electromagnetic waves, but of course, neither is exclusively one or the other. Electrons and protons are definitely particles but act in many ways like waves. And electromagnetic waves act in many ways like particles (photons), though they lack most of the special attributes that subatomic particles normally have, such as rest mass, electric charge, and so on. They’re all “wavicles”, but some are wavier than others, and of all the wavicle types, photons are the waviest.
But the degree of waviness is variable. It depends on how much energy the photon has. The more energy you put into a photon, the more it behaves like a particle.
Knowledge of electromagnetic radiation began by the study of radio. In the nineteenth century, they discovered that electric current could not only produce magnetic fields, and vice versa, but that if you rapidly oscillated an electric current, the intermittent magnetic field it created could radiate outward into space. They eventually worked out that when a momentary magnetic field collapsed, it would leave behind a bit of electric charge potential hanging freely in space, and when this collapsed, it would in turn recreate the magnetic field, but now pointed in the opposite direction. This would then collapse into another electric field, oppositely oriented from the previous one, which would reproduce the magnetic field, now right side up again. Empty space could resonate between electric and magnetic dipole fields, and this resonance would spread outward like ripples, each momentary electric or magnetic field occurring further from the source than the last, with the intensity dwindling as it spread into an ever larger volume of space.
This resonance occurs at a frequency which simply follows that of the original oscillation at the source. It might oscillate at ten thousand cycles per second, or ten million — any frequency you liked could propagate through space as freely as any other. The speed is always the same, so the frequency is inversely proportional to the wavelength — the distance covered before the oscillation returns to its original orientation. The faster the oscillation, the less amplitude of dipole field was needed to carry a given amount of energy. And the energy it carried was quite easily detectable: just put a conducting antenna into its path, and that antenna would produce a voltage which oscillated at the same frequency as the source. A receiver circuit could be tuned to preferentially receive one particular frequency, and on this basis, it is possible for many different radio transmissions to all be broadcast through the same volume of air, and for receivers to select out the particular one they want to pay attention to, while ignoring the others. This became the basis for AM radio broadcasts, and later for FM and TV and cellular towers and wifi and so on.
James Clerk Maxwell worked out the exact mathematics for how these electromagnetic waves propagated, as a set of four differential equations. It was perhaps the crowning achievement of nineteenth century science. And at around the same time, people began to strongly suspect that visible light was simply a form of radio wave, acting at a frequency far to high for any manmade apparatus to detect. And yes, that’s exactly what it is. By diffracting light through slits and so on, it became possible to measure its wavelength as a radio wave, and therefore determine its frequency, which is around half a quadrillion cycles per second — less for red light, more for blue.
But there were one or two niggling problems. Maxwell’s equations could describe exactly everything that light did when encountering normal optical situations... but it could not explain why, when it was absorbed by certain metals, the result could sometimes be a tiny electric current. Light, it turns out, could knock electrons loose from a metal surface into the air, but radio waves could not. This was called the Photoelectric Effect, and some variant of it is the basis for every sort of electronic detector of light, from the thingy that stops a closing elevator door when you wave your hand in it, to the image sensor in a TV camera. Upon further study it was found that it’s also the basis of how photographic film works, and how animal eyes work. They all depend on individual atoms getting their electrons detached, or at least loosened into an excited state, by absorbing light.
Visible light is, in a small and weak way under the most favorable conditions, ionizing radiation. It just barely qualifies. And it’s because of this that it’s the basis of life on Earth. It has just enough ionizing capability to be able to be harnessed usefully by organic molecules, so plants can harvest energy from it. It can transform those molecules into an energetic state which allows them to create foods such as sugars, without destroying the absorbing molecule in the process.
But how can an electromagnetic wave ionize anything? Maxwell’s equations, which seemed to describe the behavior of radio and light waves perfectly in other ways, said it should not happen.
It was Max Planck who inadvertently found the answer, without realizing it. Near the end of the nineteenth century, he was trying to account for the spectrum of light emitted by glowing hot materials, which could not be reconciled with the spectrum predicted by electromagnetic theory. He finally came up with a formula which worked, but it contained a strange assumption, which he hoped somebody else would be able to clear up later. This new assumption was that light of a given frequency, or wavelength, could only be emitted or absorbed in amounts of a particular size — that the amount of energy in a wave of a particular color must be a multiple of some specific base quantity, which was the minimum amount which could have that wavelength. In short, light is quantized. The ratio between the frequency and the quantized energy amount is now known as Planck’s Constant, and it’s a number that appears in almost every modern formula of quantum physics.
Albert Einstein was next to tackle the problem, and it was for this work, not for relativity, that he won the Nobel Prize. It was he who first described light as consisting of photons. The interesting question of how a set of particles could behave like a wavefront would keep physicists busy for some time, especially as they soon discovered that other particles, such as electrons, would also act like waves when passing through slits and so on. Many doubted that photons could be physically real, but Arthur H. Compton proved they were with collisions between electrons and x-rays.
When it comes to light — or to give it its full name, electromagnetic radiation — the difference between ionizing and nonionizing radiation is the amount of energy carried by an individual photon. If it’s enough so that one photon can knock one electron loose from an atom, the light is ionizing. Some materials are much more ionizable than others, so the dividing line is indefinite, but basically, everything more energetic than visible light is ionizing, and everything less energetic is not. The ionizing side, in order of increasing energy, consists of ultraviolet, x-rays, and gamma rays; as you go up, they get increasingly dangerous to your health. The other side, in order of decreasing energy, consists of infrared, microwaves, and radio. Each of those six terms denotes a very broad category which can usefully be subdivided into narrower bands, such as the UV-A, UV-B, and so on which you hear mentioned in discussion of ultraviolet.
As you go up the scale, each photon becomes more powerful and its impact more destructive. Ultraviolet can knock surface electrons off of most atoms, x-rays can knock out even the deepest core electrons, and gamma rays pack so much punch that some of them can shatter an atomic nucleus, which is untouchable by anything less. In the other direction, infrared photons are barely able to induce any changes to electrons or to chemicals, and as you move down toward microwaves, individual photons become pretty much undetectable.
Microwaves and radio waves essentially cannot be treated as individual photons; they have effect only in the mass aggregate, where they behave like electromagnetic waves as Maxwell understood them. In them you see the wave-particle duality at its most purely wavelike. (It’s for this reason that I cringe a bit when I hear the phrase “microwave radiation” — it’s a correct description, but it makes people think of microwaves as if they were ionizing and dangerous, when they’re not.)
Conversely, with gamma photons it makes far more sense to think of them as particles: they can easily be tighter and solider than many real particles, such as electrons, because some of them contain more energy than an electron does, and have a shorter wavelength and a more definite single location. When a gamma photon hits a surface, it hits a very particular point with little uncertainty about it, and it’s rare for them to fly together in such large numbers that it makes sense to talk about the wave pattern they form as a group.
Visible light and ultraviolet are in between, and show the wave-particle duality at its most even-handed — the waviness and particlishness (particularity?) are about equally prominent, with neither able to put the other into a secondary role. And the moment of absorption by a solid particularly highlights the mystery of that duality, and brings us to the core of a major unresolved question of quantum mechanics.
When a photon propagates through space, it cannot really be said to have a location specified any more precisely than its wavelength. You could say that its apparent length (as a wave) is also roughly its width, in a sense. Photons do not easily fit through holes smaller than their wavelength — once the size of the opening is below about half a wavelength, they’re pretty well blocked. This is why you can make a radio-proof room (a Faraday cage) using chicken wire or window screen instead of aluminum foil, depending on the wavelength of the signals you want to block.
And this means that if you want to say exactly where on a surface a given photon of light makes its impact... you can’t. Depending on color, a visible light wavelength is about half a micron, or 1/50,000 of an inch, and if you try to point to a particular atom within that area... well, good luck: there’ll be at least a million atoms in a circle that size. And that circle isn’t the limit of where it can hit: if it’s passed through any sort of opening, there are areas far off from a straight line through the hole where it can easily be deflected through the process of diffraction. At the moment before it’s absorbed, the energy of the photon might be spread out over billions of atoms. The fact that it strikes broadly as a wavefront is verified whenever you use a mirror: the clarity of the image depends on the mirror’s flatness over that multimillion-atom scale. But with a non-reflecting surface, because that energy cannot be divided, only one atom can absorb it. All you can express is the probability of a given atom being the lucky winner — highest in the center of the million-atom patch and much lower in distant areas. As far as current theories of physics can say, it is impossible to predict which atom that will be — the selection is not deterministic.
Where it really gets mysterious is that we don’t even know whether that one atom can definitely be said to be the recipient: by some interpretations of quantum theory, every one of them absorbed it, and what we’re living in is a superimposed multiple reality in which all of them at once are each the sole recipient, and once we observe it, we ourselves enter a superimposed reality where different versions of ourselves see different atoms having been hit... but to each individual version of us, one atom was singled out from all the others.
It’s not even clear whether we can say that the photon was travelling in any specific direction away from its source, before it was absorbed.
Whether we exist in multiple versions which each see a single reality, or whether reality itself somehow turns a probability into a single definite truth by a random roll of the dice, is at this time a question whose answer lies beyond our knowledge. And all you have to do to participate in the core of that mystery is open your eyes and look at whatever’s in front of you.