Applications & Hazards

The electromagnetic spectrum is a continuous range of radiation — from radio waves with kilometre-scale wavelengths to gamma rays with wavelengths smaller than an atomic nucleus. Every band carries energy proportional to its frequency, described by E = hf, where h is Planck's constant. Because the energy per photon varies enormously across the spectrum, different bands interact with matter in fundamentally different ways, giving each region a distinct set of uses and risks. Applications span every corner of modern life. Radio waves carry broadcasts and Wi-Fi signals. Microwaves heat food and relay phone calls via satellites. Infrared radiation is used in thermal cameras and TV remotes. Visible light drives photosynthesis and human vision. Ultraviolet light sterilises medical equipment. X-rays reveal internal anatomy. Gamma rays treat cancer and probe nuclear structure. Understanding which frequency band to use — and why — requires knowing how photon energy relates to the physical processes being exploited. Hazards arise when photon energies are high enough to damage biological tissue. The critical threshold is ionisation: removing an electron from an atom by supplying energy greater than its ionisation energy (typically 10–35 eV for biological molecules). Radiation above this threshold is called ionising radiation and includes UV-C, X-rays, and gamma rays. Below this threshold, radiation can still cause harm through heating — microwaves warming tissue is the classic example. Appreciating both mechanisms is essential for safe use of EM technology.

Key takeaways
  • Radio Waves (λ > 1 mm, f < 300 GHz)
  • Microwaves (1 mm – 1 m, 300 MHz – 300 GHz)
  • Infrared (700 nm – 1 mm, 300 GHz – 430 THz)
  • Visible Light (400–700 nm, 430–750 THz)
  • Ultraviolet (10–400 nm, 750 THz – 30 PHz)
  • X-Rays (0.01–10 nm, 30 PHz – 30 EHz)
  • Gamma Rays (λ < 0.01 nm, f > 30 EHz)

Radio Waves (λ > 1 mm, f < 300 GHz)

Radio waves have the lowest photon energies (< 1.2 meV) and easily diffract around obstacles, making them ideal for broadcasting, mobile networks, and radar. AM radio (0.5–1.7 MHz) diffracts around hills; FM (88–108 MHz) gives clearer audio over shorter distances. Wi-Fi uses 2.4 GHz and 5 GHz. At these energies photons cannot ionise atoms or even excite most molecular vibrations, so radio waves at normal intensities are biologically inert. Safety concerns at very high intensities relate to heating, but ambient broadcast power densities are thousands of times below any established threshold.

Microwaves (1 mm – 1 m, 300 MHz – 300 GHz)

Microwave photons (1.2 meV – 1.2 eV) match the rotational energy levels of polar molecules such as water. A 2.45 GHz microwave oven drives water molecules to rotate rapidly, converting electromagnetic energy into heat throughout the food — unlike a conventional oven that heats only the surface. Mobile (cell) phone signals, Bluetooth, and radar also use this band. The heating mechanism is the primary hazard: sustained intense microwave exposure can warm tissue, which is why microwave ovens have metal shielding and safety interlocks. Regulatory limits on phone radiation (SAR — Specific Absorption Rate, W kg⁻¹) are set far below levels that would cause measurable warming.

Infrared (700 nm – 1 mm, 300 GHz – 430 THz)

IR photons (1.2 meV – 1.8 eV) excite molecular vibrations and are perceived as heat. Applications include thermal imaging cameras used in firefighting and medicine, night-vision equipment, fibre-optic communications (1310 nm and 1550 nm 'telecom windows'), remote controls, and spectrometers that identify chemical bonds. The hazard is localised heating: intense IR can cause skin burns and, with prolonged exposure to near-IR, cataracts (lens clouding in the eye). Protective eyewear is mandatory when working with high-power IR lasers or furnace environments.

Visible Light (400–700 nm, 430–750 THz)

Photon energies of 1.8–3.1 eV are enough to excite electrons in pigment molecules without ionising them, enabling photosynthesis, retinal photoreception, and photochemistry. Applications include LED lighting, laser surgery (precise tissue cutting), optical fibre communications, and photography. Hazards from visible light are mainly intensity-related: staring at the sun or a high-power laser can cause permanent retinal damage because the eye's lens focuses light to a tiny spot, creating extremely high local irradiance. Laser safety classes (1 through 4) regulate permissible power levels.

Ultraviolet (10–400 nm, 750 THz – 30 PHz)

UV is subdivided into UV-A (315–400 nm), UV-B (280–315 nm), and UV-C (100–280 nm). UV-A and UV-B photons (3.1–6.2 eV) cause DNA strand breaks and pyrimidine dimers — the molecular mechanism of sunburn and, with chronic exposure, skin cancer. Ozone in the stratosphere absorbs most UV-B and virtually all UV-C reaching Earth. Melanin pigment in skin is an evolutionary response to UV damage. Beneficial applications include UV-B triggering vitamin D synthesis, UV-A enabling fluorescence detection and currency authentication, and UV-C sterilising surgical instruments by destroying microbial DNA. UV is ionising for many biomolecules at UV-C energies.

X-Rays (0.01–10 nm, 30 PHz – 30 EHz)

X-ray photons carry 120 eV – 120 keV — far above biological ionisation energies. They penetrate soft tissue readily but are absorbed by denser materials such as bone and metal, enabling radiographic imaging. Medical CT scans, airport security scanners, and crystallography (determining protein structures by diffraction) all use X-rays. Ionising damage to DNA is cumulative: each exposure carries a small but non-zero risk of inducing mutations. Radiation dose is measured in gray (Gy, J kg⁻¹) and biological effect in sievert (Sv). A chest X-ray delivers ~0.02 mSv; a CT scan ~2–10 mSv. Lead aprons and distance reduce occupational exposure.

Gamma Rays (λ < 0.01 nm, f > 30 EHz)

Gamma rays (> 100 keV, often MeV scale) originate in nuclear transitions and radioactive decay. They have the highest penetrating power of all EM radiation and are intensely ionising. In medicine, gamma-ray beams from cobalt-60 or linear accelerators are focused on tumours in radiotherapy — rapidly dividing cancer cells are more susceptible to DNA damage than healthy tissue. PET scanners detect 511 keV gamma pairs from positron annihilation to image metabolic activity. Hazards require thick concrete or lead shielding. Acute whole-body exposure above ~1 Gy causes radiation sickness; above ~6 Gy is typically fatal without medical intervention.

The Key Equation

E=hf=hcλE = hf = \frac{hc}{\lambda}
  • E — Photon energy (J (or eV)): Energy carried by one photon; determines whether it can ionise atoms or only heat matter.
  • h — Planck's constant (J·s): h = 6.626 × 10⁻³⁴ J·s; the fundamental quantum of action.
  • f — Frequency (Hz): Number of wave cycles per second; higher frequency means higher energy.
  • c — Speed of light (m s⁻¹): c = 3.00 × 10⁸ m s⁻¹ in vacuum.
  • \lambda — Wavelength (m): Distance between successive crests; inversely proportional to frequency for EM waves.

Worked Example

Problem. A medical X-ray machine uses photons with wavelength 0.050 nm. (a) Calculate the energy of one photon in joules and in eV. (b) The ionisation energy of a DNA base is approximately 8 eV. Can these X-ray photons ionise DNA? (c) If the beam delivers an absorbed dose of 0.50 mGy to 0.20 kg of tissue, how much energy is deposited?

E≈4.0×10−15 J  (24.9 keV);Edep=D×m=1.0×10−4 JE \approx 4.0 \times 10^{-15}\,\text{J} \;(24.9\,\text{keV});\quad E_{\text{dep}} = D \times m = 1.0 \times 10^{-4}\,\text{J}

Answer. Photon energy ≈ 4.0 × 10⁻¹⁵ J (24.9 keV); yes, far exceeds the 8 eV ionisation threshold; energy deposited = 1.0 × 10⁻⁴ J.

Figure
Diagram
Electromagnetic Spectrum: Applications and Hazard LevelRadio>1 mmMicro-waveInfra-redVisible400–700nmUV10–400nmX-rays0.01–10nmGamma<0.01nm← NON-IONISING | IONISING →Key ApplicationsBroadcastingWi-Fi, RadarMicrowaveovens, 5GThermalimaging, IRremotesVision,LEDs, lasersurgerySterilisationVit D, UV-AfluorescenceMedicalimaging,crystallographyRadiotherapyPET scansHazard LevelVery LowLow (heat)Low–MedLow–MedIonisingIonisingHighlyPhoton Energy (eV)10⁻⁸10⁻⁵10⁻²1.8–3.13–6120 eV–120 keV>100 keV⚠ Ionisation threshold ≈ 10 eV — radiation above this damages DNA directly
Origins · 1 minWilhelm Conrad Röntgen and how this was discovered
WC
Wilhelm Conrad Röntgen
1845–1923 · German

Röntgen discovered X-rays on 8 November 1895 while experimenting with cathode ray tubes at the University of Würzburg. He observed that a fluorescent screen across the room glowed even when the tube was covered — something the then-known cathode rays could not cause. He named the unknown radiation 'X-Strahlen' (X-rays). Within weeks he had produced the first medical X-ray image — of his wife Anna Bertha's hand — and published a paper that electrified the world. He was awarded the very first Nobel Prize in Physics in 1901. His discovery almost immediately transformed medical diagnosis and launched the field of radiation physics. Röntgen refused to patent his discovery, believing it should benefit all of humanity freely. He donated his Nobel Prize money to the University of Würzburg. His wife reportedly said of the X-ray image of her hand, 'I have seen my death!' — an eerily prescient comment given the later understanding of ionising radiation hazards.

  • Wave Properties of Electromagnetic Waves — Understanding wavelength, frequency, and speed underpins why different spectral bands have different energies and therefore different applications and hazards.
  • Photon Energy and Planck's Equation — E = hf directly determines whether a photon can ionise an atom, making it the fundamental equation connecting the spectrum to its biological effects.
  • Ionising Radiation and Dose — Gray and sievert units quantify absorbed dose and biological risk from ionising EM radiation such as X-rays and gamma rays.
  • The Electromagnetic Spectrum — A full overview of the spectrum's structure provides essential context for why each band occupies its frequency range.