All seven types of electromagnetic radiation travel at 3 × 10⁸ m/s in a vacuum but differ in wavelength, frequency, and energy per photon. The shorter the wavelength, the higher the energy: radio waves are safe at normal intensities, while gamma rays are ionising and can cause cancer.
What properties do all electromagnetic waves share?
All electromagnetic (EM) waves:
- Travel at the speed of light in a vacuum: c = 3 × 10⁸ m/s.
- Are transverse waves — oscillations are perpendicular to the direction of travel.
- Can travel through a vacuum (unlike sound, which needs a medium).
- Obey the wave equation: v = fλ (wave speed = frequency × wavelength).
- Carry energy from source to detector.
The electromagnetic spectrum (longest wavelength to shortest):
Radio → Microwave → Infrared → Visible → Ultraviolet → X-ray → Gamma
As wavelength decreases (moving right), frequency increases and photon energy increases. High-energy photons (ultraviolet and above) are ionising — they carry enough energy to remove electrons from atoms, which can damage DNA and cause mutations.
Radio waves: uses and properties
Wavelength: 1 mm to over 100 km
Frequency: ~3 Hz to ~300 GHz (lower end)
Uses:
- Television and radio broadcasting (AM radio: ~500 kHz to 1.7 MHz; FM radio: 88–108 MHz)
- Bluetooth and WiFi communication (~2.4 GHz and 5 GHz)
- Radar: locating aircraft, ships, and weather systems
- MRI scanners (use radio waves inside a strong magnetic field to image soft tissue — no ionising radiation)
Hazards: Very long wavelength radio waves at normal intensities are not harmful. Higher-frequency radio waves (approaching microwave) may cause slight tissue heating at very high intensities, but everyday exposure does not cause measurable harm.
Microwaves: uses and properties
Wavelength: 1 mm to ~30 cm
Frequency: ~1 GHz to 300 GHz
Uses:
- Microwave ovens: water molecules absorb microwave energy (at ~2.45 GHz), vibrate faster, and heat food from within.
- Mobile phone signals (4G/5G frequencies).
- Satellite communication: microwaves travel easily through the atmosphere to and from satellites.
- Radar systems.
Hazards: Microwaves cause internal heating of tissue if absorbed by the body — the main risk is high-intensity industrial or military exposure. Consumer devices operate well within safe limits.
Infrared (IR) radiation: uses and properties
Wavelength: ~700 nm to 1 mm
Frequency: ~300 GHz to ~400 THz
Uses:
- Thermal imaging cameras (detect the IR radiation emitted by warm objects).
- Remote controls for TVs and appliances.
- Night vision equipment.
- Optical fibre communication (infrared pulses carry data at very high speed).
- Physiotherapy heat lamps.
- Grills and toasters (near-IR heating elements).
Hazards: Intense IR causes burns to skin and retinal damage if the eye is exposed at high intensities (e.g. staring at the sun). Normal IR from electronics is safe.
Visible light: uses and properties
Wavelength: ~400 nm (violet) to ~700 nm (red)
Frequency: ~4 × 10¹⁴ Hz to ~7 × 10¹⁴ Hz
Uses:
- Vision (the only part of the EM spectrum the human eye can detect).
- Optical fibre communication (laser light pulses).
- Photography and video.
- Endoscopes (optical fibres carry light into and images out of the body).
Hazards: Very intense visible light (e.g. lasers) can damage the retina permanently. Normal daylight is safe for the eye in ordinary conditions.
Ultraviolet (UV) radiation: uses and properties
Wavelength: ~10 nm to ~400 nm
Frequency: ~8 × 10¹⁴ Hz to 3 × 10¹⁶ Hz
Uses:
- Detecting forged banknotes (security features fluoresce under UV).
- Sterilising water and medical equipment (UV kills bacteria by damaging their DNA).
- Promoting vitamin D synthesis in skin.
- Treating some skin conditions (phototherapy).
- Sunbeds (controversial — high risk of skin cancer).
Hazards: UV is ionising at the high-frequency end. It causes sunburn, premature skin ageing, cataracts, and skin cancer (especially malignant melanoma) with prolonged or intense exposure. The ozone layer absorbs most incoming solar UV.
X-rays and gamma rays: uses and properties
| Property | X-rays | Gamma rays |
|---|---|---|
| Wavelength | ~0.01 nm to ~10 nm | <~0.01 nm |
| Origin | Electrons decelerated in X-ray tubes | Radioactive decay in atomic nuclei |
| Uses | Medical imaging; baggage security scanners | Cancer treatment (radiotherapy); sterilising food/equipment |
| Hazards | Ionising — can cause cancer; controlled dose essential | Highly ionising — greatest cancer risk; requires thick lead shielding |
Both X-rays and gamma rays penetrate soft tissue but are partially absorbed by denser materials such as bone and lead. In medical imaging, a low dose of X-rays produces a shadow image on a detector — bones absorb more and appear white; soft tissues absorb less and appear grey. The dose is kept as low as reasonably achievable (ALARA principle).
In radiotherapy, a focused beam of gamma rays or X-rays is directed at a tumour from multiple angles; normal tissue receives a fraction of the dose at each angle but the tumour receives the full dose at the focus, destroying cancer cells.
Summary comparison table
| Type | Wavelength | Ionising? | Key use | Key hazard |
|---|---|---|---|---|
| Radio | Longest | No | Broadcasting, MRI | Negligible |
| Microwave | — | No | Cooking, mobile phones | Tissue heating at high intensity |
| Infrared | — | No | Remote controls, thermal imaging | Burns at high intensity |
| Visible | — | No | Vision, fibre optics | Laser eye damage |
| Ultraviolet | — | Partly | Banknote detection, sterilisation | Skin cancer, cataracts |
| X-ray | — | Yes | Medical imaging | Cancer risk |
| Gamma | Shortest | Yes (most) | Radiotherapy, sterilisation | Highest cancer risk |
Frequently asked questions
Why are gamma rays more dangerous than X-rays if both are ionising?
Gamma rays have shorter wavelengths and therefore higher frequencies and higher energy per photon than X-rays. Because each photon carries more energy, gamma photons are more deeply penetrating and more likely to ionise atoms along their path, causing more DNA damage. X-rays at typical medical imaging doses deliver a small, controlled dose that causes measurably increased cancer risk but at a level judged acceptable given the diagnostic benefit. Gamma rays require far more substantial shielding (thick lead and concrete) for the same protection.
Why is microwave radiation from mobile phones not considered dangerous?
Microwaves are non-ionising — their photons do not carry enough energy to remove electrons from atoms or damage DNA directly. The only established biological effect at microwave frequencies is tissue heating, and the power output of a mobile phone (~0.2 W) is far too low to cause measurable tissue temperature increases in normal use. Extensive research over 30 years has not found consistent evidence of cancer risk from mobile phone use. This is distinct from the very high power microwaves used in industrial applications, which do pose a heating hazard.
How do optical fibres use visible light or infrared to transmit information?
A thin glass or plastic fibre can carry pulses of laser light along its entire length using total internal reflection — light bounces along inside the fibre at angles beyond the critical angle, unable to escape. Millions of pulses per second (each representing a binary 0 or 1) can travel along a single fibre at nearly the speed of light with very low signal loss. Infrared wavelengths are preferred for long-distance communication because glass fibres absorb them less than visible light, allowing signals to travel thousands of kilometres without amplification.
Why does sunscreen protect against UV but not visible light?
Sunscreen contains chemical compounds (such as oxybenzone, avobenzone, or zinc oxide) that absorb or reflect UV radiation but transmit visible light. UV photons carry enough energy to be absorbed by these molecules, which then release the energy as harmless heat. Visible photons have lower energy and are not absorbed by the sunscreen molecules in the same way — which is why sunscreen is transparent (you can still see through it). The SPF (Sun Protection Factor) rating indicates how many times longer you can stay in the sun without burning compared to unprotected skin.
For GCSE physics that predicts wave behaviour before explaining the equations — try Professor Newton at aitutors.me.