Every time you use a mobile phone, get an X-ray, feel the warmth of sunlight, or see the colours of a rainbow, you are interacting with the electromagnetic spectrum. The electromagnetic spectrum is the complete range of all electromagnetic radiation — organised by wavelength, frequency, and energy — from the enormously long wavelengths of radio waves to the vanishingly short wavelengths of gamma rays. Visible light occupies only a tiny sliver of this vast range.
All electromagnetic waves are transverse waves that travel through a vacuum at the speed of light: c = 3 × 10⁸ m/s. They differ from each other in wavelength and frequency, which together determine their energy and how they interact with matter.
The electromagnetic spectrum is the complete range of electromagnetic radiation, ordered by wavelength (or equivalently frequency and photon energy). All EM waves travel at c = 3 × 10⁸ m/s in a vacuum. The seven main regions, from longest wavelength to shortest: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.
The Wave Equation and Photon Energy
All electromagnetic waves obey the universal wave equation:
where c = 3 × 10⁸ m/s is the speed of light, f is frequency (Hz), and λ is wavelength (m). Frequency and wavelength are inversely related: higher frequency means shorter wavelength.
The energy of a photon (a quantum of electromagnetic radiation) is given by Planck's equation:
where h = 6.63 × 10⁻³⁴ J·s is Planck's constant. Higher frequency → shorter wavelength → higher photon energy. This is why gamma rays are so dangerous (very high photon energy, ionising) while radio waves pass through the human body harmlessly (very low photon energy).
The Seven Regions of the Electromagnetic Spectrum
| Region | Wavelength | Frequency (Hz) | Key uses |
|---|---|---|---|
| Radio waves | > 1 mm (up to km) | < 3 × 10¹¹ Hz | Broadcasting, Wi-Fi, MRI scanners |
| Microwaves | 1 mm – 1 m | 3 × 10⁸ – 3 × 10¹¹ Hz | Microwave ovens, radar, satellite comms |
| Infrared (IR) | 700 nm – 1 mm | 3 × 10¹¹ – 4 × 10¹⁴ Hz | Thermal imaging, remote controls, fibre optics |
| Visible light | 400 – 700 nm | 4 × 10¹⁴ – 7.5 × 10¹⁴ Hz | Human vision, photography, lasers |
| Ultraviolet (UV) | 10 – 400 nm | 7.5 × 10¹⁴ – 3 × 10¹⁶ Hz | Sterilisation, vitamin D synthesis, fluorescence |
| X-rays | 0.01 – 10 nm | 3 × 10¹⁶ – 3 × 10¹⁹ Hz | Medical imaging, security scanning, crystallography |
| Gamma rays | < 0.01 nm | > 3 × 10¹⁹ Hz | Cancer radiotherapy, sterilisation, nuclear physics |
Diagram — Electromagnetic spectrum (wavelength increases right to left)
Radio Waves
Radio waves have the longest wavelengths in the spectrum — from about 1 mm to many kilometres. They are produced by oscillating electric charges in antennas. Uses include AM/FM radio broadcasting (wavelengths of metres to kilometres), television, Wi-Fi and mobile phone networks (centimetre to metre wavelengths), and MRI (Magnetic Resonance Imaging) in hospitals. Radio waves pass through walls and the human body without ionising atoms, making them safe for continuous use.
Microwaves
Microwaves span wavelengths from about 1 mm to 1 m. In a microwave oven, waves at ~12 cm wavelength (2.45 GHz) cause polar water molecules in food to rotate rapidly, generating heat. Radar systems use microwaves to detect aircraft and ships. Satellite communication relies on microwave links because they pass through Earth's atmosphere with minimal absorption. The Cosmic Microwave Background — the thermal remnant of the Big Bang — is microwave radiation at ~1.9 mm peak wavelength.
Infrared Radiation
Infrared (IR) radiation spans wavelengths from 700 nm to 1 mm. All objects at temperatures above absolute zero emit IR radiation — the hotter the object, the more IR it emits and the shorter its peak wavelength (Wien's displacement law: λ_peak = 2.898 × 10⁻³ / T). Human bodies at 37°C emit peak IR at ~9.3 μm, visible in thermal cameras. Remote controls use near-IR (~950 nm). Fibre optic communications use near-IR at 1,310 nm and 1,550 nm.
Visible Light
The visible spectrum — the only region detectable by the human eye — spans wavelengths from approximately 400 nm (violet) to 700 nm (red). Within this range: violet (~400–450 nm), blue (~450–495 nm), green (~495–570 nm), yellow (~570–590 nm), orange (~590–620 nm), red (~620–700 nm). White light contains all these wavelengths. A prism or raindrop refracts different wavelengths by different angles (longer wavelengths refract less), spreading white light into a rainbow. The connection between visible light and the broader spectrum is covered in depth in our guide to transverse waves and the electromagnetic spectrum.
Ultraviolet Radiation
UV spans 10–400 nm. The Sun emits substantial UV; Earth's ozone layer absorbs most UV-B (280–315 nm) and virtually all UV-C (100–280 nm). UV-A (315–400 nm) reaches the surface and tans skin; UV-B in small doses triggers vitamin D synthesis. Excessive UV-B causes sunburn and DNA damage — it has enough photon energy to break DNA bonds directly. UV is used in sterilisation equipment (UV-C kills microorganisms), fluorescent lamps, and forensic analysis (fluorescent substances glow under UV).
X-rays
X-rays (0.01–10 nm wavelength) are produced when high-energy electrons decelerate rapidly (bremsstrahlung) or when electrons drop between inner atomic shells. Their high photon energies allow them to penetrate soft tissue but be absorbed by denser bone and metal — the basis of medical radiography. CT (computed tomography) scanners use rotating X-ray beams to build 3D images. X-ray crystallography, which revealed the double-helix structure of DNA and the structures of thousands of proteins, works by diffracting X-rays off crystal lattice planes.
Gamma Rays
Gamma rays (< 0.01 nm) have the highest frequencies and photon energies in the spectrum. They are produced by nuclear reactions — radioactive decay, nuclear fission, neutron capture — and by astrophysical processes such as supernovae and black hole accretion discs. Their high photon energy makes them deeply penetrating and ionising. Medical applications include PET scanning (positron emission tomography), which uses gamma rays from electron-positron annihilation (each producing two 511 keV photons), and radiotherapy, where targeted gamma rays destroy tumour cells.
All EM Waves Share These Properties
Despite their enormous range of wavelengths and energies, all electromagnetic waves share fundamental properties:
• They are transverse waves — oscillating electric and magnetic fields perpendicular to each other and to the propagation direction.
• They travel at c = 3 × 10⁸ m/s in a vacuum — the same for all regions, regardless of wavelength or frequency.
• They can travel through a vacuum — they do not require a medium (unlike sound waves).
• They carry energy — photon energy E = hf.
• They can be reflected, refracted, diffracted, and polarized.
• They obey c = fλ.
Frequently Asked Questions
The Full Spectrum
All electromagnetic radiation is oscillating electric and magnetic fields travelling at c = 3 × 10⁸ m/s in vacuum. The only difference between types is frequency (and hence wavelength: λ = c/f) and energy per photon (E = hf). From lowest to highest frequency:
| Type | Wavelength | Frequency | Key uses |
|---|---|---|---|
| Radio | >10 cm | <3 GHz | Broadcasting, MRI, radar |
| Microwave | 1 mm–10 cm | 3–300 GHz | Wi-Fi, 5G, cooking, satellite |
| Infrared | 700 nm–1 mm | 300 GHz–430 THz | Thermal imaging, remote controls, fibre optics |
| Visible light | 400–700 nm | 430–750 THz | Vision, photography, solar cells |
| Ultraviolet | 10–400 nm | 750 THz–30 PHz | Sterilisation, sunburn, vitamin D |
| X-rays | 0.01–10 nm | 30 PHz–30 EHz | Medical imaging, crystallography, airport security |
| Gamma rays | <0.01 nm | >30 EHz | Cancer treatment, nuclear medicine, sterilisation |
Properties Common to All EM Waves
All EM waves: travel at c = 3 × 10⁸ m/s in vacuum; can travel through vacuum (no medium needed); are transverse waves (oscillating E and B fields perpendicular to each other and to direction of propagation); can be reflected, refracted, diffracted, and polarised; obey v = fλ with v = c; carry momentum p = E/c = hf/c per photon; and cause ionisation if their photon energy exceeds the ionisation energy of the absorbing atoms.
The Visible Spectrum
Human eyes detect wavelengths from ~400 nm (violet) to ~700 nm (red). The colours in order: violet (400–450 nm), blue (450–495 nm), cyan, green (495–570 nm), yellow (570–590 nm), orange (590–620 nm), red (620–700 nm). White light contains all colours — prisms and raindrops separate them by dispersion (different refractive indices for different wavelengths). The sun emits most intensely around 500 nm (green-yellow) — matching the peak sensitivity of human eyes (~555 nm). Different animals see different parts of the spectrum; bees see near-UV, snakes detect infrared.
Frequently Asked Questions
What is the electromagnetic spectrum?
The electromagnetic spectrum is the complete range of electromagnetic radiation, ordered by frequency and wavelength. From lowest to highest frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. All are the same phenomenon — oscillating electric and magnetic fields — distinguished only by frequency. All travel at c = 3 × 10⁸ m/s in vacuum and obey E = hf (photon energy) and v = fλ. The boundaries between types are gradual — there is no sharp cutoff between, say, X-rays and gamma rays; the distinction is by source (atomic processes for X-rays, nuclear for gamma) rather than wavelength.
Which part of the EM spectrum has the highest energy?
Gamma rays have the highest photon energy (E = hf increases with frequency). Gamma rays from nuclear decays typically have energies of 100 keV to 10 MeV. Cosmic gamma rays can reach 100 TeV and beyond from extreme astrophysical events (neutron star collisions, black hole jets). For comparison, visible light photons have energies of 1.8–3.1 eV — about a million times less than typical nuclear gamma rays. Higher energy means greater penetrating power and ionising ability: gamma rays can penetrate metres of concrete and are the most biologically hazardous form of EM radiation.
What is ionising radiation?
Ionising radiation carries enough energy per photon to remove electrons from atoms (ionise them). The ionisation energy of atoms is typically 10–25 eV. Ultraviolet with energies above ~3 eV can ionise some molecules but not most atoms; UV-C (100–280 nm) is ionising enough to damage DNA. X-rays (keV energies) and gamma rays (MeV energies) are fully ionising and can penetrate tissue, causing DNA damage that can lead to cancer. Radio waves, microwaves, infrared, and visible light are non-ionising — their photons lack enough energy to displace electrons, though intense microwaves can heat tissue (microwave oven effect).
What is the speed of all electromagnetic waves?
All electromagnetic waves travel at c = 299,792,458 m/s in vacuum, regardless of frequency. This constancy — the same for radio waves as for gamma rays, for any observer regardless of motion — is the experimental foundation of special relativity. In materials, EM waves slow to c/n where n is the refractive index. Radio waves in glass travel at ~2 × 10⁸ m/s; visible light in diamond at ~1.24 × 10⁸ m/s. Different frequencies slow by different amounts in materials (dispersion), causing rainbows and chromatic aberration in lenses.
How are radio waves and gamma rays the same?
Radio waves and gamma rays are both electromagnetic radiation — oscillating electric and magnetic fields propagating through space. They are both transverse waves that travel at c in vacuum, can be reflected, refracted, and polarised, and are described by the same wave equation. The only difference is frequency: radio waves oscillate millions to billions of times per second; gamma rays oscillate 10²⁰ times per second or faster. This frequency difference gives them completely different photon energies (radio: ~10⁻⁷ eV vs gamma: ~MeV), penetrating ability, and interaction mechanisms with matter — hence their very different applications and biological effects.
What is the electromagnetic spectrum?
The electromagnetic spectrum is the complete range of all electromagnetic radiation, ordered by wavelength, frequency, and energy. It includes (from longest wavelength to shortest): radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. All travel at c = 3 × 10⁸ m/s in a vacuum.
What are the 7 types of electromagnetic waves?
The 7 regions of the electromagnetic spectrum are: (1) radio waves, (2) microwaves, (3) infrared, (4) visible light, (5) ultraviolet, (6) X-rays, and (7) gamma rays. These categories are not sharply bounded — the spectrum is continuous, and the boundaries are conventions.
What is the speed of electromagnetic waves?
All electromagnetic waves travel at c = 3 × 10⁸ m/s (299,792,458 m/s exactly) in a vacuum. This is the speed of light. In materials such as glass or water, EM waves slow down — the ratio of c to the wave's speed in the medium is the refractive index n. Radio waves, light, and gamma rays all travel at the same speed in vacuum.
Which electromagnetic wave has the highest frequency?
Gamma rays have the highest frequency (above ~3 × 10¹⁹ Hz), the shortest wavelength (below ~0.01 nm), and the highest photon energy. They are produced by nuclear reactions and astrophysical events and are the most penetrating and ionising form of electromagnetic radiation.
Are electromagnetic waves transverse or longitudinal?
All electromagnetic waves are transverse. The electric and magnetic field oscillations are perpendicular to the direction of wave propagation — and perpendicular to each other. This is why electromagnetic waves can be polarized (a property unique to transverse waves). Sound waves, by contrast, are longitudinal.
What is the wavelength of visible light?
Visible light spans approximately 400 nm (violet) to 700 nm (red). Within this range: violet/blue (400–495 nm), green (495–570 nm), yellow/orange (570–620 nm), red (620–700 nm). The exact boundaries vary slightly between individuals — human colour vision is determined by three types of cone cells with peak sensitivities near 420 nm, 534 nm, and 564 nm.
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