The electromagnetic spectrum
What it is
The EM spectrum is a continuous family of transverse waves that all travel at the speed of light (c = 3 × 10⁸ m/s) in a vacuum. They differ in wavelength and frequency (c = f λ).
The seven groups (long λ → short λ)
| Type | Typical use | Danger |
|---|---|---|
| Radio | TV, radio broadcast | None at low intensity |
| Microwaves | Cooking, satellite, mobile phones | Internal heating of cells |
| Infrared (IR) | Remote controls, heaters, night vision, fibre optics | Skin burns |
| Visible light | Sight, photography, fibre optics | Damage to retina at high intensity |
| Ultraviolet (UV) | Fluorescent lamps, security marks, tanning | Sunburn, skin cancer, cataracts |
| X-rays | Medical imaging, security scanners | Ionising — damage cells, cancer |
| Gamma | Cancer treatment, sterilising equipment, tracers | Most ionising — cancer, mutation |
Memorise the order: Radio, Micro, IR, Visible, UV, X-ray, Gamma.
Wave equation
c = f λ — speed of light = frequency × wavelength.
If frequency is in Hz and λ is in m, c is in m/s.
Why ionising waves are dangerous
UV, X-rays and gamma have enough energy per photon to knock electrons off atoms (ionisation). This can:
- Damage DNA → cell mutation → cancer.
- Kill living cells (used to sterilise equipment).
Lower energy waves (radio, microwave, IR) heat tissue but do not ionise.
Refraction at a boundary
EM waves slow down when entering a denser medium (e.g. light into glass) and bend towards the normal. Used in lenses, prisms and fibre optics.
Detection
Different detectors for different wavelengths: aerials (radio), thermometers (IR), photographic film (visible/UV), Geiger counter (gamma/X-ray).
OCR exam tip
If asked why microwaves cook food, the marks are for water molecules absorbing microwaves → vibrate / gain energy → heat food — not "they cook it because they're hot".
AI-generated · claude-opus-4-7 · v3-ocr-combined-science-leaves