Electromagnetic Waves: Light Without a Medium
Right now, invisible ripples made of pure electric and magnetic fields are passing through your body — carrying this page to your screen, warming your skin from 150 million kilometres away, and letting you see these very words. No medium, no wires, no strings attached.
A Wave With Nothing to Wave In
For most of history, waves needed something to travel through. Sound needs air — which is why in films, explosions in space are silent even if Hollywood disagrees. Water waves need water. So when physicists realised light was a wave, a huge question appeared:
If light is a wave... what is it waving through?
Scientists invented a hypothetical invisible substance called the "luminiferous aether" (light-carrying ether) that supposedly filled all of space. But careful experiments — most famously the Michelson–Morley experiment (1887) — tried to detect Earth moving through this aether and found absolutely nothing.
The resolution came from James Clerk Maxwell (1860s), who unified electricity and magnetism into four elegant equations. His equations predicted that changing electric fields create magnetic fields, and changing magnetic fields create electric fields — so the two could regenerate each other endlessly, flying through empty space as a self-sustaining wave. When Maxwell calculated its speed from known electrical measurements, he got about m/s — exactly the measured speed of light. Light was an electromagnetic wave.
This is one of the great unifications in science: electricity, magnetism, and optics turned out to be three faces of one thing.
Anatomy of an Electromagnetic Wave
An electromagnetic wave is a transverse wave: the disturbances wiggle at right angles to the direction the wave travels. But unlike a wave on a string, there are two wiggles happening at once:
- An electric field (E) oscillating in one direction
- A magnetic field (B) oscillating at 90° to it
Both are also at 90° to the direction of travel, and they rise and fall in step — peak together, zero together. The wave carries energy in the direction of E × B.
Like any wave, we describe it with:
- Wavelength (λ) — distance between successive peaks, measured in metres
- Frequency (f) — how many complete oscillations pass a point each second, measured in hertz (Hz)
- Amplitude — the height of the field oscillation; related to the wave's intensity/brightness
where is the speed of light. This is just the universal wave equation () applied to light — but because every electromagnetic wave travels at the same speed in a vacuum, wavelength and frequency are locked together: high frequency always means short wavelength.
Polarisation is a bonus consequence of being transverse: since the fields oscillate in specific directions, we can filter waves by orientation. Polarising sunglasses exploit this to cut glare from horizontal surfaces like water and roads.
EM waves are transverse: the electric and magnetic fields oscillate perpendicular to each other and perpendicular to the direction of travel. They oscillate in phase, and they need no medium.
One Spectrum, Many Colours
Maxwell's theory and later experiments showed that all electromagnetic waves — from enormous radio waves to tiny gamma rays — are the same kind of thing, differing only in frequency and wavelength. Laid out in order, they form the electromagnetic spectrum:
| Band | Wavelength (approx.) | Typical source / use |
|---|---|---|
| Radio | > 1 m | TV, radio broadcasting, astronomy |
| Microwaves | 1 mm – 1 m | Ovens, radar, Wi-Fi, mobile phones |
| Infrared (IR) | 700 nm – 1 mm | Heat radiation, remote controls, thermal imaging |
| Visible light | 400 – 700 nm | Vision, fibre optics, lasers |
| Ultraviolet (UV) | 10 – 400 nm | Sunburn, sterilising equipment, fluorescence |
| X-rays | 0.01 – 10 nm | Medical imaging, airport security |
| Gamma rays | < 0.01 nm | Radioactive decay, cancer treatment |
Notice that visible light — the only part we can see — is a razor-thin slice of the whole spectrum. Higher frequency means higher photon energy, which is why UV and above can damage cells (sunburn, cancer risk), while radio waves pass through us harmlessly. New Zealand's sunburn risk is largely due to strong UV, partly because of the ozone hole's historical effect over the Southern Ocean.
Roughly how many times wider is the radio band than the entire visible band?
The visible range spans roughly 400–700 nm, while radio wavelengths exceed a metre — a factor of over a million. On a log scale, visible light occupies less than one 'octave' out of more than twenty.
The Universal Speed Limit
In a vacuum, every electromagnetic wave travels at the same speed: m/s m/s.
That's fast enough to circle Earth about 7.5 times per second. Because , changing the frequency forces the wavelength to change so the speed stays fixed. A radio station broadcasting at 100 MHz emits waves of wavelength:
In materials, light slows down. In water it moves about 25% slower; in glass about 33% slower. The ratio is called the refractive index of the material. This slowing is what makes a straw look bent in a glass of water — light changes speed and therefore direction at the boundary (refraction).
Since c = fλ and c is fixed, halving the wavelength doubles the frequency. Speed never changes in a vacuum.
How EM Waves Interact With Matter
Electromagnetic waves interact with matter in four main ways, and different bands do different things:
- Transmission — the wave passes through (glass passes visible light; flesh passes X-rays fairly well)
- Absorption — the wave's energy is soaked up, usually heating the material (microwaves excite water molecules; your skin absorbs infrared as warmth)
- Reflection — the wave bounces off (metals reflect visible light — that's why they shine; ionised layers in the upper atmosphere bounce some radio waves around Earth)
- Refraction — the wave bends when changing speed between materials (lenses, prisms, rainbows)
This selectivity is why the spectrum is so useful: X-rays pass through soft tissue but are absorbed by bone, giving us medical images. Microwaves penetrate cloud but radio telescopes on the ground can still catch them. And greenhouse gases let visible sunlight through but absorb outgoing infrared — the basic mechanism of climate change.
Whether an EM wave is transmitted, absorbed, reflected, or refracted depends on both the wavelength and the material. Exploiting this selectivity gives us X-ray imaging, microwave ovens, optical fibres, and radio communication.
Communication technology is essentially applied spectrum management: your phone juggles several microwave bands at once, Wi-Fi uses crowded 2.4 and 5 GHz channels, and regulators (like NZ's Ministry for Regulation of radio spectrum) auction off slices of the spectrum to broadcasters and telecom companies — because two transmitters on the same frequency interfere with each other.
Summary
- Electromagnetic waves are oscillating electric and magnetic fields that regenerate each other and need no medium.
- They are transverse: E ⊥ B ⊥ direction of travel, oscillating in phase.
- All EM waves travel at m/s in vacuum, linked by .
- The electromagnetic spectrum orders them by frequency: radio → microwaves → infrared → visible → ultraviolet → X-rays → gamma rays. Higher frequency = higher energy.
- Matter interacts selectively with different bands via transmission, absorption, reflection, and refraction — the basis of imaging, cooking, communications, and climate physics.
Hertz didn't think his discovery was useful
Heinrich Hertz first generated and detected radio waves in 1887, deliberately confirming Maxwell's prediction. Asked about applications, he reportedly said "It's of no use whatsoever... this is just an experiment that proves Maestro Maxwell was right." Within a decade, Marconi had sent radio signals across the English Channel.