How Does a Light Switch Fill a Room With Light?
Note: these posts are AI-assisted explorations written for my own understanding, not as definitive reference material. If you spot any inaccuracies, I'd love to hear about them.
You flip a light switch. The room fills with light. You see.
Half a second. That’s all it took. A thought in your brain, a movement of your finger, and photons are streaming from a thin wire, crossing five meters of empty space at 299,792,458 meters per second, entering your eyes, triggering molecular machinery in your retinas, and sending electrical signals racing to your brain.
Five different things happened. A thought. A movement. Light. Chemistry. Perception.
They’re the same thing.
Not metaphorically. Not loosely connected. The nerve signal that told your finger to move is electromagnetic. The current in the wire is electromagnetic. The light from the bulb is electromagnetic. The molecule in your retina that catches the photon responds electromagnetically. The signal that races back to your brain along your optic nerve is electromagnetic.
One force. Talking to itself. Start to finish.
We’re going to trace every step.
The Journey
Here’s everything that happens between your finger and your brain:
Note: The highlighted node marks where we are in the journey right now.
Part 1: The Circuit
Q1: You flip a light switch. What physically happens at the switch?
Your finger pushes a plastic lever on the wall. Inside the housing, a piece of metal swings across a 3 mm air gap and touches another piece of metal.
That’s it. The entire mechanism. A piece of metal bridges a gap.
Note: The metal contacts either touch (closed circuit) or separate (open gap).
Source: Wikimedia Commons (public domain).
But consider what that gap was doing. Air is an insulator; it resists the flow of electric charges. Copper is a conductor; charges move through it freely. The difference between these two materials is not a matter of degree. The resistivity of air is about 10¹⁶ ohm-meters. The resistivity of copper is 1.7 × 10⁻⁸ ohm-meters.
The ratio is 10²⁴. A trillion trillion.
Flipping the switch does not gradually improve the connection. It obliterates a barrier. One moment, the path from the power source to the light bulb is broken by a wall of resistance a trillion trillion times greater than the wire. The next moment, that wall is gone. A continuous loop of conducting material; a circuit; now connects the power source to the bulb and back.
If the switch contacts don’t fully close; if they corrode, or if the screw loosens; the connection point becomes a resistor. Current still flows, but the contact point itself heats up. Sometimes enough to melt plastic. Sometimes enough to start a fire. The U.S. Consumer Product Safety Commission estimates roughly 67,800 electrical fires per year. Many start at a connection that was almost closed.
Q2: The circuit is complete. What flows?
The conducting loop is unbroken. Something begins to move. What?
Electrons. Every copper atom has 29 electrons, but one of them is loosely bound; it can wander freely through the metal’s crystal structure. These are free electrons. In a cubic meter of copper, there are approximately 8.5 × 10²⁸ of them.
That number is hard to feel. Here it is written out: 85,000,000,000,000,000,000,000,000,000. In a space the size of a sugar cube. They were there before you flipped the switch, drifting randomly, going nowhere in particular.
When the circuit closes, they begin drifting in one direction. Not fast; we’ll get to that. But together.
Current is measured in amperes. One ampere means one coulomb of charge passing a point each second; roughly 6.24 × 10¹⁸ electrons. A typical 60-watt incandescent bulb draws about 0.5 amperes. That’s 3 × 10¹⁸ electrons through the filament every second.
Three quadrillion electrons per second. Through a wire thinner than a human hair. And you notice nothing.
But what makes them move?
Q3: What IS electric charge? Why do electrons move at all?
Charge is a property of matter, as fundamental as mass. Just as mass determines how strongly something responds to gravity, charge determines how strongly it responds to the electromagnetic force.
There are two types. We call them positive and negative. Protons carry positive charge. Electrons carry negative charge. Neutrons carry none. The interaction rule is absolute: like charges repel; opposite charges attract.
This attraction and repulsion is the electromagnetic force; one of four fundamental forces in the universe, alongside gravity, the strong nuclear force, and the weak nuclear force.
Here’s a number worth sitting with. Compare the electromagnetic force between two electrons to the gravitational force between the same two electrons.
The electromagnetic force is stronger by a factor of 4.2 × 10⁴².
That is 42 orders of magnitude. Forty-two zeros. The electromagnetic force is so overwhelmingly dominant at the atomic scale that gravity is essentially irrelevant. Every chemical bond in your body, every nerve signal in your brain, every photon striking your retina; all electromagnetic. Gravity holds you to the floor. Electromagnetism does nearly everything else.
Charge has three properties that matter for our journey:
It is quantized; it comes in indivisible packets. You cannot have half an electron’s charge. Every observed charge is an exact integer multiple of e ≈ 1.602 × 10⁻¹⁹ coulombs.
It is conserved; it is never created or destroyed, only moved. The total charge of the universe is constant.
It is invariant; the charge of a particle is the same regardless of how fast it moves. (Mass changes with velocity, as Einstein showed. Charge does not.)
So what makes the electrons move through the wire? The power source; a generator at a power plant, or a battery; maintains a charge imbalance. More electrons at one terminal, fewer at the other. This imbalance creates an electric field throughout the circuit, and that field pushes on every free electron.
But what IS an electric field?
Q4: What IS an electric field?
A charge does not reach across space and pull on another charge. Nothing touches. Instead, every charge modifies the space around itself. Any other charge that enters that modified space feels a force.
The modification of space IS the electric field.
The field at any point is the force that would act on a unit positive charge placed there: Here E is the electric field, F is the force on the test charge, and q is that charge.
Read it as: the electric field at a point is “how much push per unit charge” a positive test charge would feel there.
The field radiates outward from every charge and falls off with the square of distance: For a single source charge Q at distance r, and with ε₀ the permittivity of free space, the field magnitude is:
Read it as: the field from one charge gets weaker fast—double the distance and the field becomes four times smaller.
Numerically, ε₀ ≈ 8.854 × 10⁻¹² C²/(N·m²); it governs how strongly charges interact.
Note: Field lines show direction and strength; denser lines mean a stronger field.
Is this field real? This was not an academic question. For decades, most physicists believed the field was a mathematical convenience; a bookkeeping device. Michael Faraday insisted it was a real physical entity. He could see it, he said. In the patterns iron filings made around magnets, in the way charges distributed themselves on conductors. The field was there.
Faraday was a bookbinder’s apprentice with no formal mathematical training. Most of the scientific establishment did not take his intuition seriously.
He was right.
We know the field is real because it carries energy. Electromagnetic waves transport energy through empty space; from the Sun to your skin, from a radio tower to your phone. It carries momentum; light exerts pressure on objects (radiation pressure). And it propagates at a finite speed; changes in the field travel at the speed of light, not instantaneously.
A mathematical convenience cannot carry energy or push on things. The field is as real as the charge that creates it.
If the field propagated infinitely fast, there would be no electromagnetic waves. No light. No radio. No way for information to travel through space. The finite speed of field propagation is essential to everything that follows.
Q5: What pushes the electric field through the wire? What IS voltage?
The power source; a generator at a power plant, or a battery; does work to separate charges. It pushes electrons to one terminal (making it negative) and pulls them away from the other (making it positive). This creates an imbalance.
Note: Transformers step voltage up for transmission and down for safe household use.
Source: Wikimedia Commons (CC0).
The measure of that imbalance is voltage. Voltage is energy per unit charge; the amount of energy given to each coulomb of charge that passes through the source. Here V is voltage, W is energy (work), and q is charge.
One volt means one joule per coulomb. Your US wall outlet provides 120 volts. A European outlet: 230 volts. A high-voltage transmission line: up to 765,000 volts.
When the circuit closes, this voltage establishes an electric field throughout the conducting path. The field points from high potential to low; from the positive terminal through the circuit to the negative terminal. Electrons, being negative, drift the opposite way; toward the positive terminal.
The relationship between voltage, current, and resistance was discovered by Georg Simon Ohm in 1827: Here I is current and R is resistance.
Resistance measures how much a material opposes current. A 60-watt bulb’s filament has a resistance of about 240 ohms when hot. One meter of copper wire: about 0.005 ohms.
If a low-resistance path bypasses the filament; a nail falls across the terminals, a wire insulation cracks; current surges. At 120 volts through near-zero resistance, current can reach hundreds of amperes. The wire heats instantly. This is a short circuit. A typical household breaker trips at 15 to 20 amperes. Below that threshold, the wire is on its own.
Note: We’ve completed the switch and are now inside the circuit.
Q6: The light comes on almost instantly. But electrons move incredibly slowly. What gives?
You flip the switch. The light appears. Maybe a tenth of a second, maybe less.
The wire from your breaker panel to the light fixture is about 15 meters long. If the electrons carry the energy, they need to travel that distance. So: how fast do electrons move through copper?
Let’s calculate. For a 12 AWG household copper wire carrying 0.5 amperes: Here v_d is drift speed, I is current, n is the number of free electrons per cubic meter, A is the wire’s cross‑sectional area, and e is the charge of one electron.
Read it as: the same current drifts more slowly when there are more electrons available or when the wire is thicker.
That is 0.011 millimeters per second. Slower than a snail by a factor of a thousand.
At this speed, an electron would take 16 days to travel 15 meters.
The light comes on in a fraction of a second. The electrons carrying the current would take two weeks to reach the bulb.
Something else is happening.
When the switch closes, the change in the electric field propagates through and around the wire at roughly two-thirds the speed of light; about 2 × 10⁸ m/s. This field change reaches the filament in: Here t is the travel time.
Seventy-five billionths of a second. The field arrives at the filament and tells the electrons already there; the 8.5 × 10²⁸ per cubic meter that were sitting in the tungsten all along; to start moving. They don’t need to travel from the switch. They were already at the filament. The field just gave them a push.
Sixteen days. Versus 75 nanoseconds. The same wire. Two completely different velocities. One is matter drifting. The other is a field propagating.
Note: Compare the slow drift of electrons with the fast-moving field signal.
A telephone call across the Atlantic; 5,500 km; takes about 27 milliseconds at the speed of the field. If the signal traveled at electron drift speed, the same call would take roughly 14 million years.
Note: We’re now at the “field propagation” step.
Q7: Energy reaches the filament. What happens?
The field arrived in 75 nanoseconds. Electrons in the filament begin to drift.
The filament is a thin coil of tungsten wire; about 0.05 mm in diameter, roughly 50 cm long if you uncoiled it. Tungsten is chosen for one reason: it has the highest melting point of any metal. 3,422°C.
It needs to survive what comes next.
Note: The thin coil maximizes surface area so it can glow intensely without melting.
Source: Wikimedia Commons (CC0).
Each free electron is accelerated by the electric field. Within about 10⁻⁸ meters; roughly the spacing between atoms; it collides with a tungsten atom in the crystal lattice. The collision transfers the electron’s kinetic energy to the atom, making it vibrate harder. The electron slows down, gets accelerated again, and hits another atom. Over and over.
Billions of electrons. Billions of collisions per second. Each collision deposits a tiny amount of energy. The cumulative effect of all this atomic vibration is what we call heat. This is resistive heating; the conversion of electromagnetic field energy into thermal energy. Here P is power, I is current, V is voltage, and R is resistance.
For a 60-watt bulb: 60 joules of electromagnetic energy become 60 joules of thermal energy every second. The filament heats to about 2,700 K in roughly a tenth of a second.
Hot enough to glow.
At operating temperature, tungsten atoms slowly evaporate from the filament surface and deposit on the glass envelope; that dark coating you see on old bulbs. The filament thins gradually, over about 1,000 hours, until it breaks. If a voltage spike pushes the filament above 3,422°C, the tungsten melts and the bulb dies immediately.
Edison’s first commercial bulbs used carbonized bamboo filaments. They lasted 1,200 hours but couldn’t run as hot. Earlier experiments used platinum, osmium, even human hair. Tungsten won because it could survive the temperatures needed to produce visible light.
And producing visible light is what matters. Because the filament is now white-hot. And something is about to happen that classical physics cannot explain.
Q8: The filament is white-hot. Why does a hot object emit light?
You are glowing right now.
Not metaphorically. Every object above absolute zero; 0 kelvin, −273.15°C; emits electromagnetic radiation. Your body, at about 310 K, radiates at a peak wavelength of roughly 9,300 nanometers. Deep infrared. Your eyes cannot detect it, but an infrared camera can. You are a thermal radiator, glowing in wavelengths you were never built to see.
Your coffee mug is glowing. The walls of your room are glowing. The chair you’re sitting in. Everything. All the time.
Why? Because the atoms in any material are vibrating, and those atoms contain charged particles; protons and electrons. A vibrating charge is an accelerating charge. And an accelerating charge radiates electromagnetic energy. (This is a consequence of Maxwell’s equations; which we will arrive at later. For now, the fact: accelerating charges radiate.)
As temperature rises, two things happen. The total radiation increases; not linearly, but as the fourth power of temperature: Here P is radiated power per unit area, T is absolute temperature, and σ is the Stefan–Boltzmann constant.
Read it as: total radiated power rises extremely fast with temperature—double T and the power jumps by 16×.
Double the temperature: sixteen times the radiation. And the peak wavelength shifts shorter: Here λ_max is the wavelength where the emission peaks, b is Wien’s displacement constant, and T is temperature.
Read it as: hotter objects peak at shorter (bluer) wavelengths; cooler objects peak at longer (redder/infrared) wavelengths.
| Temperature | Peak Wavelength | What You See |
|---|---|---|
| 310 K (your body) | ~9,300 nm | Nothing visible; deep infrared |
| 800 K (dull red heat) | ~3,600 nm | Faintest red glow |
| 1,300 K (cherry red) | ~2,200 nm | Steady red glow |
| 2,700 K (filament) | ~1,070 nm | Warm yellow-white |
| 5,778 K (Sun’s surface) | ~500 nm | White; peak in green |
The filament at 2,700 K peaks in the near-infrared; still invisible. But the emission spectrum is broad. Its tail extends into the visible range; wavelengths between 380 and 700 nanometers; and that tail is what you see.
Only about 5% of the filament’s total radiation falls in the visible band. The rest is infrared; heat you can feel on your face from across the room, but cannot see. An incandescent bulb is a heater that happens to also produce a small amount of light.
Note: As temperature rises, the curve shifts left (bluer) and grows taller (more power).
The shape of this emission curve; how much radiation at each wavelength; follows a universal function that depends only on temperature. It does not depend on the material. A tungsten filament, an iron bar, and a ceramic pot at the same temperature all produce the same spectrum. This universal curve is called the blackbody spectrum.
Explaining its exact shape would trigger a revolution in physics. Because when physicists tried to derive this curve from classical thermodynamics, they got a prediction that worked at long wavelengths; and then diverged to infinity at short ones.
Note: The heat-and-glow step is where the filament becomes a light source.
Part 2: The Birth of Quantum Physics
Q9: Classical physics predicts infinite radiation at short wavelengths. What went wrong?
Your coffee cup is at about 350 K. According to classical thermodynamics, it should be emitting infinite energy.
Not a lot of energy. Infinite.
The logic seems airtight. The equipartition theorem; a cornerstone of classical physics; says every possible mode of vibration in a warm object carries the same average energy. At shorter wavelengths, there are more modes. More modes, same energy each, total energy climbs without limit.
Lord Rayleigh and James Jeans derived the spectrum from these principles in 1900. Their formula; the Rayleigh-Jeans law; matches experiment perfectly at long wavelengths. At short wavelengths, it diverges. Not a small error. Not a factor of two. The prediction heads to infinity.
Your coffee cup should be blasting you with ultraviolet radiation. And X-rays. And gamma rays. It isn’t.
Paul Ehrenfest later named this the ultraviolet catastrophe. It was not a minor discrepancy. The most fundamental theories of physics; mechanics, electromagnetism, and thermodynamics; combined correctly, applied honestly, producing a prediction that was infinitely wrong.
Note: The classical curve blows up at short wavelengths; the quantum curve stays finite.
If classical physics had been correct, the catastrophe would be literal. Every object above absolute zero would radiate infinite energy, instantly cooling to 0 K while flooding the universe with lethal radiation. Stable matter at finite temperature would be impossible. The fact that your coffee cup does not kill you is evidence that classical physics is incomplete.
Q10: How did Planck solve the ultraviolet catastrophe?
On December 14, 1900, Max Planck presented a paper to the German Physical Society. He made one change to the classical derivation.
Note: Planck introduced energy quanta in 1900, igniting quantum theory.
Source: Wikimedia Commons (public domain).
Energy is not continuous. It comes in packets.
A vibrating atom can only emit or absorb energy in discrete units; quanta; of size:
Where h is a new constant (6.626 × 10⁻³⁴ J·s, now called Planck’s constant) and f is the frequency of the radiation.
This single assumption eliminates the catastrophe. At high frequencies, each quantum carries a lot of energy (because E = hf and f is large). To emit even one quantum at very high frequency requires more energy than thermal fluctuations can supply. The high-frequency modes are not forbidden. They are priced out. The cost of admission for a single quantum is more than most atoms can pay.
At low frequencies, quanta are cheap. Many are emitted. The Rayleigh-Jeans prediction is recovered.
The resulting curve; the Planck distribution; matches experiment perfectly at all wavelengths and all temperatures, with no adjustable parameters beyond h.
Planck called his assumption “an act of desperation.” He did not believe energy was really quantized. He thought it was a mathematical trick that would eventually be explained by classical physics. He spent years trying to derive the same result without quantization.
He failed.
He later wrote: “I was ready to sacrifice any of my previous convictions about physics.”
At 2,700 K, the thermal energy (0.23 eV) can easily excite infrared quanta (0.12 eV) but struggles to excite visible-light quanta (2.5 eV). This is why most of the filament’s radiation is infrared. Not because infrared is preferred. Because visible quanta cost too much.
If Planck’s constant were zero, energy would be continuous, and we’d be back to the ultraviolet catastrophe. If it were much larger, even infrared quanta would require enormous energy. Room-temperature objects would be invisible even in infrared. The value of h; 6.626 × 10⁻³⁴; is the quantization of nature.
Note: This marks the transition from classical physics to quantization.
Q11: Einstein took this further. What did he propose?
Planck quantized the emission and absorption of energy. But he still thought of light itself as a continuous wave. The packets were a rule about how atoms exchange energy; not about the light traveling between them.
In 1905, Einstein made a bolder claim. Light itself is quantized. It consists of particles, each carrying energy E = hf. Not just the emission. Not just the absorption. The light itself comes in packets. (Gilbert Lewis named them photons in 1926.)
Note: Einstein explained the photoelectric effect by treating light as photons.
Source: Wikimedia Commons (public domain in the U.S.).
Einstein’s evidence: the photoelectric effect. When light shines on a metal surface, electrons are sometimes ejected. This had been observed since 1887; by Heinrich Hertz, ironically, while verifying Maxwell’s wave theory. But the results were puzzling.
Classical wave theory predicted three things. Brighter light should eject faster electrons. Any frequency should work if you make the light bright enough. There should be a time delay as the wave’s energy accumulates in the electron.
All three predictions were wrong.
Brighter light ejects more electrons, but they are no faster. Below a certain threshold frequency, no electrons are ejected at all, no matter how intense the light. Above the threshold, electrons are ejected instantly; within 10⁻⁹ seconds; even with extremely dim light.
Einstein’s explanation: each photon delivers energy E = hf. To eject one electron, one photon must deliver at least the work function φ; the minimum energy to free an electron from the metal surface. Here K_max is the maximum kinetic energy of the ejected electron.
If hf < φ: nothing happens, regardless of brightness. Each photon lacks enough energy, and photons do not pool their resources. If hf > φ: instant ejection, even with dim light. One photon is enough.
Note: Increase frequency to cross the threshold; intensity changes count, not energy per photon.
Robert Millikan spent ten years performing precision photoelectric measurements, explicitly intending to disprove Einstein’s equation. His data confirmed it to high precision instead. The value of h he measured agreed with Planck’s value to within 1%.
Einstein received the Nobel Prize in 1921. Not for relativity. For this.
Note: Photons are now part of the story.
Q12: A photon leaves the 2,700 K filament. What is its energy? What color is it?
Back to our filament. It is emitting photons across a broad spectrum according to the Planck distribution. The peak wavelength, from Wien’s law:
This peak is in the near-infrared. Invisible. The filament is not producing a single color. It is producing a continuous spectrum that slopes downward from red to violet:
| Wavelength | Color | Relative intensity |
|---|---|---|
| 1,073 nm | Infrared (invisible) | 100% (peak) |
| 700 nm | Deep red | ~72% |
| 600 nm | Orange-yellow | ~56% |
| 450 nm | Blue | ~29% |
Because the spectrum slopes from red to blue, the light appears warm yellowish-white; enriched in red and orange, deficient in blue compared to sunlight.
A visible photon at 600 nm carries energy: Here E is photon energy, h is Planck’s constant, c is the speed of light, and λ is wavelength.
A 60-watt bulb converts about 5% of its power into visible photons. That is roughly 3 watts of visible light:
Eight billion billion visible photons per second. And several times more infrared photons, wasted as heat.
To shift the peak into the visible range (550 nm), you would need a temperature of about 5,270 K. Tungsten melts at 3,695 K. No filament material can survive the temperature needed for efficient visible radiation. This is a fundamental thermodynamic limit; not an engineering failure. It is why LEDs; which produce light through a non-thermal mechanism; have largely replaced incandescent bulbs.
Part 3: What IS Light?
Q13: A photon leaves the filament and crosses the room. What IS it doing as it travels?
This is one of the deepest questions in physics.
A photon is an oscillating electromagnetic disturbance; a coupled pair of electric and magnetic fields vibrating in sync as they travel through space. The electric field oscillates in one direction; say, up and down. The magnetic field oscillates perpendicular to it; left and right. Both are perpendicular to the direction of travel. The fields regenerate each other: the changing electric field creates the magnetic field, and the changing magnetic field creates the electric field.
No medium is required. Unlike sound (which needs air) or ocean waves (which need water), electromagnetic waves propagate through perfect vacuum. The fields sustain themselves.
But this is only half the answer.
A photon is also quantized. It has a definite energy (E = hf) and momentum (p = h/λ). Here p is momentum. It is detected at a single point; whole photon or nothing. But it diffracts. It interferes. Single photons fired one at a time at a double slit still produce an interference pattern over many trials; something only a wave can do.
It is not a particle. It is not a wave. It is something with no classical analogue. Wave-particle duality is not a compromise or an approximation. It is the fundamental nature of light; and of all quantum objects.
Note: E and B fields oscillate perpendicular to each other and to the direction of travel.
For our journey, the key fact: the photon crosses your 5-meter room at 299,792,458 m/s. That takes about 17 nanoseconds. In those 17 nanoseconds, a green photon (550 nm) completes roughly 9,000 oscillations of its electric and magnetic fields.
Note: We’re following the photon across the room.
Q14: You keep saying “magnetic field.” What IS a magnetic field?
A charge sitting still creates only an electric field. The moment it moves, something new appears.
A magnetic field wraps around the direction of motion in circular loops. Its strength falls off as 1/r from the wire. And it does something an electric field alone cannot: it exerts a force only on moving charges, and that force is perpendicular to both the velocity and the field. Here F is force, q is charge, v is velocity, and B is the magnetic field; “×” indicates a direction perpendicular to both v and B.
A magnetic field never speeds up or slows down a charge. It only deflects. It curves trajectories without changing speeds.
Current in a wire creates a circular magnetic field around it. Electrons orbiting in atoms create tiny magnetic fields; this is what makes permanent magnets. Earth’s liquid iron core, convecting and rotating, creates Earth’s magnetic field.
If moving charges did not create magnetic fields, electric motors would not work. Generators would not work. Electromagnetic waves could not exist. There would be no light.
Q15: A current creates a magnetic field. Can a magnetic field create a current?
Ørsted showed that electricity creates magnetism. Does the reverse work?
Yes. Michael Faraday discovered in 1831 that a changing magnetic field induces an electric current in a nearby conductor. Not a static field; it must be changing. Move a magnet through a coil of wire, and current flows. Stop the magnet, and the current stops.
Note: Faraday discovered induction and made fields a physical reality.
Source: Wikimedia Commons (public domain).
The keyword is changing.
Faraday’s law: the induced voltage around a closed loop equals the negative rate of change of magnetic flux through that loop. Here ℰ (emf) is the induced voltage, Φ_B is the magnetic flux through the loop, and t is time.
Read it as: change the magnetic flux through a loop, and the loop pushes back with a voltage that opposes that change.
The negative sign (Lenz’s law) means the induced current opposes the change that caused it; a manifestation of energy conservation. Push a magnet into a coil, and the induced current creates a magnetic field that pushes back against the magnet. You have to do work to move it. That work becomes electrical energy. Nothing is free.
Note: Move the magnet to change flux and watch the induced current reverse direction.
Every electric generator on Earth works on this principle. The turbines in power plants; spun by steam, water, or wind; rotate magnets past coils of wire, converting mechanical rotation into electric current. The power plant that lit your bulb uses Faraday’s discovery from 1831.
If electromagnetic induction did not work, there would be no generators, no transformers, no alternating current. Batteries and solar cells would be the only electrical sources. Long-distance power transmission would be impossible. The entire electrical infrastructure of civilization depends on this.
Q16: Changing magnetic fields create electric fields. Can changing electric fields create magnetic fields?
In the 1860s, James Clerk Maxwell was working to unify all known laws of electricity and magnetism into a single mathematical framework. When he examined the existing equations, he found a problem.
Note: Maxwell unified electricity and magnetism and predicted electromagnetic waves.
Source: Wikimedia Commons (public domain).
Ampère’s law said magnetic fields are created by currents. But in certain situations; specifically, a charging capacitor where current flows in the wires but not between the plates; the law gave contradictory results depending on which mathematical surface you used to evaluate it. The equations were inconsistent.
Maxwell’s fix: he added a term. Not only do actual currents create magnetic fields, but changing electric fields also create magnetic fields. He called this the displacement current, though no actual charge is displaced.
This was a theoretical prediction. No experiment had demonstrated it yet. Maxwell added it because without it, the mathematics broke.
The symmetry was now complete. Changing B creates E (Faraday, 1831). Changing E creates B (Maxwell, 1865).
Q17: Changing E creates B, and changing B creates E. What does this imply?
A self-sustaining wave.
Oscillate a charge. The oscillating charge creates a changing electric field. That changing electric field creates a magnetic field. But this magnetic field is now changing, because the electric field is still oscillating. The changing magnetic field creates a new electric field. Which creates more magnetic field. And so on.
The disturbance leapfrogs outward through empty space. Each field regenerates the other. No wire. No medium. No material of any kind. Self-sustaining.
Maxwell derived the speed of this wave from his equations. It depends on two constants; both measurable in the laboratory with nothing more than charges, currents, and rulers: Here μ₀ is the magnetic constant (permeability of free space) and ε₀ is the electric constant (permittivity of free space).
Read it as: the speed of light is set by two electrical constants—change those, and light’s speed would change too.
Maxwell knew the speed of light. Fizeau had measured it in 1849. Foucault refined the measurement in 1862. The value: approximately 3.15 × 10⁸ m/s.
Two numbers. One from electrostatics and magnetism. One from optics. They had no reason to be related.
They were the same.
In 1865, Maxwell wrote one of the most extraordinary sentences in the history of science:
“The agreement of the results seems to show that light and magnetism are affections of the same substance, and that light is an electromagnetic disturbance propagated through the field according to electromagnetic laws.”
Light is an electromagnetic wave. Electricity, magnetism, and optics; three seemingly separate branches of physics; are one phenomenon.
If E and B did not regenerate each other, electromagnetic disturbances would die out as they traveled. No electromagnetic waves. No light. No radiation of any kind. The Sun’s energy could not reach Earth.
Q18: What are Maxwell’s four equations?
Four equations. The complete classical theory. In these equations, ∇· (divergence) describes how a field spreads out from a point, ∇× (curl) describes how a field swirls, ρ is charge per volume, and J is current per area.
- Gauss’s law for electricity: Electric field lines originate from charges. ∇·E = ρ/ε₀
- Gauss’s law for magnetism: There are no magnetic monopoles. Magnetic field lines always form closed loops. ∇·B = 0
- Faraday’s law: A changing magnetic field creates an electric field. ∇×E = −∂B/∂t
- Ampère-Maxwell law: Currents and changing electric fields create magnetic fields. ∇×B = μ₀J + μ₀ε₀∂E/∂t In plain English: charges are sources of E; magnetic field lines never start or end; changing B makes swirling E; and currents or changing E make swirling B.
These four equations, plus the Lorentz force law (F = qE + qv × B), describe every electromagnetic phenomenon at the classical level. Every circuit. Every radio wave. Every photon crossing your room.
Note: The four equations are the compact rules for how E and B are sourced and swirl.
Each equation was discovered separately. Coulomb and Gauss established the first. Nobody has ever found a magnetic monopole, establishing the second. Faraday found the third in 1831. Ampère found half the fourth in 1826; Maxwell completed it in 1865.
Richard Feynman wrote: “From a long view of the history of mankind; seen from, say, ten thousand years from now; there can be little doubt that the most significant event of the 19th century will be judged as Maxwell’s discovery of the laws of electrodynamics.”
Not the American Civil War. Not the Industrial Revolution. Maxwell’s equations.
Remove Gauss’s electric law and charges don’t create fields; no electrostatics, no chemistry. Remove Faraday’s law and there is no induction; no generators, no power grid. Remove the displacement current and there are no electromagnetic waves; no light. Each equation is independently necessary.
Note: We’re at the unification of electricity, magnetism, and light.
Q19: What else travels as an electromagnetic wave?
Everything.
Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays; all electromagnetic waves, differing only in frequency. The spectrum spans over 20 orders of magnitude:
| Region | Wavelength | Photon Energy | Sources |
|---|---|---|---|
| Radio | > 1 m | < 1.2 μeV | AM/FM radio, TV |
| Microwave | 1 mm – 1 m | 1.2 μeV – 1.2 meV | WiFi, cell phones, microwave ovens |
| Infrared | 700 nm – 1 mm | 1.2 meV – 1.8 eV | Warm objects, remote controls |
| Visible | 380 – 700 nm | 1.8 – 3.3 eV | Stars, fires, bulbs, LEDs |
| Ultraviolet | 10 – 380 nm | 3.3 – 124 eV | Sun, sterilization lamps |
| X-ray | 0.01 – 10 nm | 124 eV – 124 keV | Medical imaging |
| Gamma ray | < 0.01 nm | > 124 keV | Radioactive decay, supernovae |
There are no hard boundaries. The spectrum is continuous. “Microwave” blends into “infrared” blends into “visible.” The categories are human conventions.
Your body emits infrared. Your router emits microwaves. The Sun emits across the entire spectrum. A dental X-ray machine and a radio station are doing the same thing at vastly different frequencies.
Note: Visible light is a tiny slice of the full electromagnetic spectrum.
Visible light occupies less than one millionth of one percent of the electromagnetic spectrum by wavelength range. We see this sliver and nothing else.
Why can we only see 380–700 nm? Because that is where the Sun’s emission peaks (Wien’s law at 5,778 K gives λ_max ≈ 500 nm), and because water; which fills our eyes; is transparent in this range. Evolution tuned our eyes to the brightest available signal through the most transparent available medium.
Part 4: The Deepest Level
Q20: Is there a level deeper than Maxwell’s equations? How do charges REALLY interact?
Yes.
Quantum electrodynamics; QED; developed in the late 1940s by Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga (independently, all three sharing the 1965 Nobel Prize), describes the electromagnetic force not as a continuous field pushing on charges, but as the exchange of virtual photons between charged particles.
Two electrons do not simply push each other apart through a static field. They exchange photons; particles of the electromagnetic field itself. The “force” is the cumulative effect of countless photon exchanges, calculated using Feynman diagrams that track every possible path the photons could take.
These virtual photons are not the photons you detect with a camera. They exist briefly, permitted by the Heisenberg uncertainty principle, which allows energy to be “borrowed” for short times. The more energy borrowed, the shorter the photon can exist, the shorter the distance it can travel. This is why the electromagnetic force weakens with distance.
The strength of this photon exchange is governed by a single number: the fine structure constant, α ≈ 1/137. It measures how strongly charges couple to photons. The fact that α is small (much less than 1) is why QED calculations converge; each additional virtual photon exchange contributes roughly 1/137 times less than the previous.
At everyday scales, QED reproduces Maxwell’s equations exactly. The classical “field” emerges as the statistical average of astronomical numbers of virtual photon exchanges. The field is real; but it arises from a deeper quantum reality.
Q21: How accurate is QED?
Classical physics (the Dirac equation, 1928) predicts that the electron’s magnetic moment has a value of exactly g = 2. QED says it should be slightly more than 2, because the electron is constantly emitting and reabsorbing virtual photons that modify its magnetic behavior.
The QED prediction, computed to 5th order in perturbation theory (requiring the evaluation of 12,672 Feynman diagrams):
The experimentally measured value, from a single electron confined in a Penning trap:
Agreement to better than one part in a trillion. Twelve significant digits.
This is like predicting the distance from New York to Los Angeles to within the width of a human hair. It is like predicting the age of the universe to within half a second.
No other theory in any field of science has been tested to this precision and survived.
There is one tantalizing crack. The muon; a heavier cousin of the electron; shows a g − 2 value that may disagree with QED at the 4–5 sigma level, reported by the Muon g − 2 experiment at Fermilab. If confirmed, it could indicate physics beyond QED. This is one of the most active areas of current research.
Note: This is the deepest, quantum-level explanation.
Q22: Is electromagnetism connected to any other forces?
In the 1960s, Sheldon Glashow, Abdus Salam, and Steven Weinberg showed that electromagnetism and the weak nuclear force are two aspects of a single electroweak force, unified at energies above about 100 GeV (roughly 10¹⁵ K). Below that energy, a process called spontaneous symmetry breaking gives the weak force carriers (the W and Z bosons) their large masses while leaving the photon massless. The unity breaks. The two forces appear different.
This was confirmed in 1983 when the W and Z bosons were discovered at CERN, with exactly the masses the theory predicted. The Higgs boson; the particle associated with the symmetry-breaking mechanism; was found at CERN in 2012.
Note: This situates electromagnetism among the other three fundamental forces.
The four fundamental forces: strong nuclear, electromagnetic, weak nuclear, and gravity. The strong force holds quarks together inside protons and holds protons together in nuclei. The weak force governs certain radioactive decays and is essential for stellar fusion. Gravity shapes the large-scale structure of the universe.
A refrigerator magnet lifting a paperclip is overcoming the gravitational pull of the entire Earth. Gravity is 10³⁶ times weaker than electromagnetism. It only dominates at large scales because matter is electrically neutral in bulk; positive and negative charges cancel, but mass never cancels.
Part 5: The Return Path
Q23: The photon crosses the room and enters your eye. What happens?
The photon has traveled 5 meters in 17 nanoseconds. It enters through the cornea; the clear, curved front surface of the eye, which does about 65% of the focusing. It passes through the aqueous humor, through the pupil (the adjustable opening in the iris), through the crystalline lens (which fine-tunes the focus), through the vitreous humor, and arrives at the retina; a thin layer of light-sensitive cells, roughly 0.2 mm thick, at the back of the eye.
Note: Light enters from the left and reaches the retina at the back after passing through the lens.
Source: Wikimedia Commons (CC BY-SA 4.0).
The retina contains about 120 million rod cells (sensitive to dim light, no color) and 6 million cone cells (sensitive to color, less sensitive overall). The retina is built backwards; light must pass through several layers of neurons before reaching the photoreceptors at the back. A historical accident of vertebrate evolution.
Our photon, at roughly 600 nm from a tungsten filament, is absorbed by a rod cell. The entire transit through the eye took about 0.1 nanoseconds.
Note: The photon has reached the eye.
Q24: The photon hits a rod cell. What happens at the molecular level?
Inside each rod cell are approximately 100 million molecules of rhodopsin; a protein with a small molecule called retinal (derived from vitamin A) embedded in it. Retinal has a specific bent molecular shape. When a photon is absorbed, its energy causes a precise structural change: the retinal straightens. From 11-cis to all-trans. In about 200 femtoseconds.
200 × 10⁻¹⁵ seconds. One of the fastest known chemical reactions.
This tiny shape change triggers a massive amplification cascade:
| Step | What happens | Amplification |
|---|---|---|
| 1 | Retinal straightens; opsin changes shape | 1 rhodopsin activated |
| 2 | Activated rhodopsin activates transducin | ~100 transducin molecules |
| 3 | Each transducin activates PDE (phosphodiesterase) | ~100 PDE molecules |
| 4 | Each PDE breaks down cGMP | ~1,000 cGMP per PDE |
| 5 | Without cGMP, sodium channels close | ~250 channels close |
| 6 | Fewer Na⁺ ions entering the cell | ~1 million fewer Na⁺/s |
One photon. 200 femtoseconds. A million ions. The cell’s voltage changes by about 1 millivolt. A nerve signal fires.
Note: Each step multiplies the signal, turning one photon into a large electrical response.
This cascade is why your eyes can detect single photons. Experiments confirm that a dark-adapted human can perceive a flash of as few as 5–7 photons reaching the retina; perhaps 1–2 actually absorbed by rod cells.
Without vitamin A, the body cannot synthesize retinal. Without retinal, rhodopsin cannot form. Without rhodopsin, rod cells are blind. This is why vitamin A deficiency causes night blindness; one of the oldest known nutritional diseases, described in ancient Egyptian medical texts around 1500 BCE.
Q25: A nerve signal fires toward the brain. What IS a nerve signal?
An action potential. A brief, self-propagating wave of voltage change along a nerve cell.
At rest, a neuron maintains −70 millivolts across its membrane; inside negative relative to outside. Protein pumps continuously move Na⁺ out and K⁺ in, holding this resting potential.
When triggered, voltage-gated sodium channels open at one point. Na⁺ rushes in. The voltage spikes from −70 mV to about +40 mV. This spike triggers the next section of membrane to open its sodium channels. The signal propagates like a wave. Behind the wave, potassium channels open, K⁺ flows out, restoring the resting potential. The whole cycle at each point takes about 1–2 milliseconds.
This is not an electric current flowing through the nerve like current through a wire. It is a wave of ion channel openings; a domino chain of electromagnetic events propagating along the fiber.
In bare nerve fibers, this wave travels at about 1 m/s. But the optic nerve is wrapped in myelin; a fatty insulating sheath with periodic gaps. The signal jumps between gaps (saltatory conduction), reaching about 50 m/s. Distance from retina to brain: about 15 cm. Transit time: about 3 milliseconds.
Note: The spike is a traveling voltage change, not a bulk flow of electrons.
Multiple sclerosis strips the myelin sheath from nerve fibers. Without it, signal speed drops from ~50 m/s to ~1 m/s, and many signals are lost entirely. Vision problems are often the first symptom.
Note: The signal is now traveling through the nervous system.
Q26: The signal reaches your visual cortex. You perceive light. But wait; what just happened?
Let’s trace back through every step:
| Step | What happened | Force responsible |
|---|---|---|
| Brain fires signal to hand | Action potential (ion channels) | Electromagnetic |
| Finger presses switch | Contact force (electron repulsion) | Electromagnetic |
| Metal contacts close | Electrons flow through conductor | Electromagnetic |
| Field propagates through wire | Electric field at ⅔c | Electromagnetic |
| Filament heats | Electron-atom collisions | Electromagnetic |
| Filament emits photons | Thermal radiation | Electromagnetic |
| Photon crosses room | EM wave propagation | Electromagnetic |
| Cornea and lens focus light | Refraction (EM interaction with matter) | Electromagnetic |
| Rhodopsin absorbs photon | Molecular shape change | Electromagnetic |
| Ion cascade amplifies signal | Chemical signaling | Electromagnetic |
| Nerve signal travels to brain | Action potential | Electromagnetic |
| Visual cortex processes signal | Neural electromagnetic activity | Electromagnetic |
Every row. Every single one.
Part 6: The Revelation
Q27: So almost everything we experience is electromagnetic. What ISN’T?
Three things.
Gravity holds you to your chair, keeps the Earth orbiting the Sun, and shapes the large-scale structure of the universe. You feel it constantly. But every other sensation; sight, sound, touch, taste, smell; is electromagnetic.
The strong nuclear force holds quarks together inside protons and neutrons, and holds protons and neutrons together inside atomic nuclei. Without it, nuclei would fly apart (protons repel each other electromagnetically), and atoms heavier than hydrogen could not exist. No carbon. No oxygen. No life.
The weak nuclear force governs certain types of radioactive decay and is essential for the fusion reactions that power the Sun. It is already unified with electromagnetism at high energies (the electroweak force).
Remove electromagnetism and you have no atoms, no light, no chemistry, no biology, no technology, no perception. Remove gravity and you have no planets or stars. Remove the strong force and you have no nuclei. Remove the weak force and the Sun goes out.
All four appear necessary for a universe with complexity, chemistry, and life.
Q28: What IS electromagnetism?
One force. The interaction between charged particles, mediated by the photon, responsible for virtually everything you have ever seen, felt, heard, or experienced.
At the deepest level; a quantum field theory in which charged particles exchange virtual photons. The most precisely verified theory in the history of science.
At the classical level; four equations that unify electric fields, magnetic fields, and light into a single framework.
At the human level; the force that holds your body together, carries signals through your brain, illuminates your world, and powers every technology you have ever used.
It was discovered in pieces over two centuries; by Franklin, Coulomb, Volta, Ørsted, Ampère, Faraday, Maxwell, Hertz, Planck, Einstein, Dirac, Feynman, and hundreds of others. Each piece seemed separate. Each was a different face of the same force.
The Complete Loop
You flipped a switch. Light filled the room. You saw.
The nerve signal from your brain to your finger; electromagnetic. The current through the wire; electromagnetic. The field racing at two-thirds the speed of light; electromagnetic. The filament glowing white-hot; electromagnetic. The photon crossing five meters of empty space; electromagnetic. The rhodopsin molecule catching it; electromagnetic. The cascade of a million ions; electromagnetic. The signal racing back along your optic nerve; electromagnetic. The visual cortex assembling it all into the experience of light; electromagnetic.
Every step. Every single one. The same force talking to itself.
Total time from decision to perception: about half a second.
Note: This is the time budget from your decision to your perception.
Note: The loop is complete; we’re back at human perception.
Electromagnetism at Every Level
| Layer | What electromagnetism IS at this layer |
|---|---|
| Your experience | Light, color, warmth, touch, sight, thought |
| Biology | Nerve signals, photoreception, molecular cascades |
| Chemistry | Bonds between atoms, molecular structure, reactions |
| Classical physics | Electric and magnetic fields; Maxwell’s four equations |
| Quantum physics | Photon exchange between charged particles (QED) |
| Fundamental | One of four forces; unified with weak force (electroweak) |
| Universal | The force responsible for all atomic structure, all light, all chemistry |
Further Reading
-
Maxwell, J.C., “A Dynamical Theory of the Electromagnetic Field” (1865); The paper that unified electricity, magnetism, and light. Dense but historic.
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Feynman, R.P., QED: The Strange Theory of Light and Matter (1985); The best popular explanation of quantum electrodynamics, by one of its creators. No equations. Pure insight.
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Griffiths, D.J., Introduction to Electrodynamics; The standard undergraduate textbook. Clear, rigorous, and surprisingly readable.
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Schoenlein, R.W. et al., “The First Step in Vision” (1991), Science 254; The femtosecond spectroscopy paper that resolved the speed of retinal photoisomerization.
