Electromagnetic radiation

• 1. Introduction

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  Electromagnetic radiation reaches your eye from a star whose light left it before you were born. That light carries dark bands, gaps where atoms in the star's atmosphere swallowed certain frequencies and scattered them away. Read those gaps and you can name the chemical elements burning inside a sun you will never visit. The same family of waves lets a radio station broadcast without spending more power when a million extra listeners tune in. It lets a snake feel the warmth of prey in the dark. It can cook food, burn skin, treat cancer, and ionize the atoms it strikes. Every one of these waves moves through empty space at one fixed speed, 299,792,458 meters per second, roughly 186,000 miles per second. What is the thing that does all of this? It behaves as a wave you can reflect, refract, and bend through a prism. It also behaves as a stream of particles with no mass at all. How a single phenomenon can be both, and how a handful of equations predicted light itself, is the story ahead.

• 2. Wave Or Particle

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  Wave-particle duality means electromagnetic radiation shows wave traits and particle traits at the same time, and both have been confirmed in many experiments. The wave nature dominates when you measure the radiation over large distances and long stretches of time. The particle nature shows itself over tiny distances and short timescales. When the average number of photons in a cube of the relevant wavelength drops far below one, the lumpy, particle-like deposit of energy becomes hard to miss.

  A single photon sent through an interferometer makes the paradox vivid. At low intensity, a sensitive detector or photomultiplier registers the light along only one arm of the device, exactly as a particle would. Yet let many such detections pile up and an interference pattern emerges, exactly as a wave would. The same lone photon seems to take both paths and one path at once.

  The Copenhagen interpretation says the act of observation truly collapses the entity's wave function into a single outcome. The many-worlds interpretation says every possible outcome happens, each in its own parallel universe. Pilot wave theory says the particle's behavior is simply steered by an underlying wave. The double-slit experiment has shown this dual nature for a real photon, leaving physicists to argue not about the result but about what it means.

• 3. The Equations That Predicted Light

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  James Clerk Maxwell derived a wave form of the electric and magnetic equations and uncovered a hidden symmetry between the two fields. The speed his wave equation predicted matched the measured speed of light so closely that Maxwell concluded light itself must be an electromagnetic wave. Between 1862 and 1864 he set down equations suggesting that visible light, and by inference invisible infrared and ultraviolet rays, were all traveling disturbances in the electromagnetic field.

  Four equations carry the theory. Two of them Maxwell refined from earlier work: Faraday's Law of Induction and Ampere's circuital law, to which he added a term of his own called the displacement current. He pictured that displacement current as the motion of bound charges, and believed it gave rise to the magnetic field. The remaining pair are Gauss's law and Gauss's law for magnetism.

  In homogeneous, isotropic media these equations force the radiation to be a transverse wave. The electric field and the magnetic field both stand perpendicular to the direction the wave travels, and perpendicular to each other. Their strengths hold a fixed ratio, and in lossless media they stay in phase, reaching their peaks and troughs at the same points in space. Heinrich Hertz later confirmed the whole picture through his experiments with radio waves, turning Maxwell's mathematics into something he could generate and detect on a bench.

• 4. Near And Far

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  Maxwell's equations split the field around a source into two parts that behave nothing alike. Currents make magnetic fields of a dipole type, and charges pushed apart in an antenna make an electric-dipole field, but both die away quickly with distance. These make up the near field, and they do not radiate. They transfer energy only to a receiver very close by, as inside a transformer. The near field tugs back on its own source. Draw energy from it and the load on the transmitter rises. If nothing absorbs it, the near field simply oscillates and hands its energy back to the transmitter.

  The far field is the part that breaks free. A radio station needs the same power to push its signal outward whether one receiver or a thousand are listening. Once the far field leaves the transmitter, its existence and its energy are wholly independent of both transmitter and receiver. This free, radiating part is what we call electromagnetic radiation.

  Distance tells the two fields apart by how fast they fade. The power density of radiation from an isotropic source falls off with the inverse square of the distance, the inverse-square law. The dipole parts of the near field fade faster, by an inverse-cube law. In the Lienard-Wiechert formulation for a single moving particle, only the terms tied to the particle's acceleration produce true radiation. The terms from its steady velocity belong to the near field and carry no energy away.

• 5. The Birth Of The Photon

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  An anomaly haunted physics in the late 19th century, a clash between the wave theory of light and the spectra pouring out of thermal radiators called black bodies. The problem resisted solution for years and earned a name: the ultraviolet catastrophe. In 1900 Max Planck broke it open with a new theory of black-body radiation. His idea was that black bodies emit light only in discrete bundles of energy, which he called quanta.

  Albert Einstein took the next step in 1905, proposing that these light quanta be treated as real particles. The particle of light was later named the photon, to sit alongside the electron and proton being described in the same era. A photon's energy is proportional to its frequency through the Planck constant, a relation sometimes called the Planck-Einstein equation. Its momentum is likewise proportional to frequency and inversely proportional to wavelength.

  Einstein's evidence came from the photoelectric effect, an anomaly the wave theory could not explain. Light striking a metal surface ejected electrons and drove a current. The energy of each ejected electron tracked the frequency of the light, not its intensity. Below a minimum frequency that depended on the metal, no current flowed at all, no matter how bright the beam. Because the wave theory was so well supported, established physicists met Einstein's particle idea with great skepticism. Acceptance came only as more particle-like behavior turned up, such as the Compton effect.

• 6. Reading The Light Of Atoms

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  When a photon is absorbed by an atom, it lifts an electron to a higher energy level, on average farther from the nucleus. When that electron falls back to a lower level, it emits a photon whose frequency matches the energy gap exactly. Because the energy levels in atoms are discrete, every element and every molecule emits and absorbs its own characteristic set of frequencies. Immediate re-emission is fluorescence, a kind of photoluminescence, seen when fluorescent paint glows under ultraviolet blacklight. Delayed emission is phosphorescence.

  A glowing gas reveals the same machinery. Heat or any other mechanism excites the atoms, and as electrons drop down they emit a spectrum of sharp lines marking the jumps between levels. The emission spectrum of nebulae is one example. Rapidly moving electrons are accelerated most sharply when they hit a region of force, which makes them the source of much of the highest-frequency radiation found in nature.

  These spectra are a fingerprint that can be read across cosmic distances. Spectroscopy determines which chemical elements make up a particular star. A shift of an element's spectral lines toward longer wavelengths, a redshift, can be turned into the star's cosmological distance. The dark bands in a distant star's light are the work of atoms in that star's own atmosphere, absorbing certain frequencies and scattering them out of the beam.

• 7. Why Frequency Changes Everything

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  Classified by wavelength, electromagnetic radiation runs from radio waves longer than a continent to gamma rays smaller than the nucleus of an atom. Frequency rises as wavelength shrinks, and higher frequency means higher-energy photons. No fundamental limit is known at either end of this spectrum, though photons near or above the Planck energy, far higher than any ever observed, would demand new physics to describe.

  At radio and microwave frequencies, the radiation works on matter as a bulk crowd of charges spread across many atoms. Radio waves induce currents on a conductor's surface by moving electrons in correlated bunches, the basis of antennas. Microwaves are absorbed by water and other molecules carrying an electric dipole moment, the principle behind a microwave oven. Infrared instead grips the dipoles inside single molecules, jostling atoms at the ends of a chemical bond, which is why so many substances absorb it and warm up.

  Visible light, between roughly 400 and 700 nanometers, is the narrow band the human eye detects directly. Its photons carry just enough energy to change the bond structure of certain molecules. That is no accident, since vision itself depends on a single molecule, retinal, absorbing a single photon and flipping from cis to trans, which reshapes the rhodopsin protein around it and triggers the retina to sense light. Photosynthesis works in the same band, where one photon excites one molecule of chlorophyll.

  Push into the ultraviolet and photons carry about three electron volts or more, enough to force doubly bonded molecules into permanent rearrangement and to inflict lasting damage on DNA. Near 10 electron volts, with wavelengths below 124 nanometers, photons can tear electrons clean off atoms in photoionization. From the extreme ultraviolet upward, every X-ray and gamma ray counts as ionizing radiation, capable of the most severe molecular damage, including mutation and cancer, often deep beneath the skin.

• 8. Heat, Harm, And The Heat Ray

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  Bioelectromagnetics studies how electromagnetic radiation acts on living organisms, and the effect depends on the radiation's power and frequency. From radio waves up to near ultraviolet, the best-understood harm comes from power alone, through heating as the radiation is absorbed. Frequency still matters, because it sets how deeply the radiation penetrates; microwaves reach further into tissue than infrared. Fields too weak to cause meaningful heating are widely accepted to have no biological effect, though some research hints that weak modulated radiofrequency and microwave fields might, with significance still unclear.

  The World Health Organization has classed all ultraviolet frequencies as Group 1 carcinogens. Ultraviolet from the sun is the primary cause of skin cancer, the most common cancer among fair-skinned people, damaging DNA and blocking its repair. Radiofrequency radiation sits in Group 2B, possibly carcinogenic, a category that also holds lead, coffee, and automobile exhaust.

  The destructive end of this story has been weaponized. The heat ray uses microwave frequencies to create an unpleasant heating effect in the upper skin, realized in the Active Denial System, an experimental device built by the US military to deny an enemy access to an area. A death ray is the theoretical version, electromagnetic energy strong enough to injure tissue. Harry Grindell Matthews, working in the 1920s on a death ray built around a microwave magnetron, claimed he lost the sight in his left eye to it. The discovery of these waves began far more gently. In 1800, William Herschel passed sunlight through a glass prism and found invisible calorific rays beyond the red, raising a thermometer's temperature. Those rays were later named infrared, the first hint that the spectrum stretched far past anything the eye could see.