The Electromagnetic Spectrum: Wavelengths and Frequencies

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 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

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