P6.2 Electromagnetic Waves
The electromagnetic spectrum
All electromagnetic (EM) waves:
- Travel at 3 × 10⁸ m/s in a vacuum (the speed of light, c)
- Are transverse waves
- Can travel through a vacuum
- Transfer energy
The EM spectrum in order of increasing frequency (decreasing wavelength):
| Type | Wavelength (m) | Frequency (Hz) | Notes |
|---|---|---|---|
| Radio waves | >0.1 | <3×10⁹ | Longest λ, lowest f |
| Microwaves | 10⁻³–0.1 | 3×10⁹–3×10¹¹ | |
| Infrared | 10⁻⁶–10⁻³ | 3×10¹¹–4×10¹⁴ | |
| Visible light | 4×10⁻⁷–7×10⁻⁷ | 4×10¹⁴–7×10¹⁴ | Red→violet |
| Ultraviolet | 10⁻⁸–4×10⁻⁷ | 7×10¹⁴–3×10¹⁶ | |
| X-rays | 10⁻¹¹–10⁻⁸ | 3×10¹⁶–3×10¹⁹ | |
| Gamma rays | <10⁻¹¹ | >3×10¹⁹ | Shortest λ, highest f, most energy |
Mnemonic: "Really Massive Iguanas Violently Urge eXtra Grapefruit"
Applications of EM radiation
| Type | Application |
|---|---|
| Radio | Broadcasting, WiFi, Bluetooth |
| Microwave | Satellite communication, cooking, radar |
| Infrared | Thermal imaging, remote controls, optical fibres (short range), night vision |
| Visible | Photography, sight, fibre optic communications |
| Ultraviolet | Sterilisation, fluorescence, detecting forged banknotes |
| X-rays | Medical imaging of bones, airport security |
| Gamma | Cancer treatment (radiotherapy), sterilising medical equipment |
Hazards of EM radiation
Higher frequency → shorter wavelength → more energy per photon → more ionising → more damaging to cells:
| Type | Risk |
|---|---|
| Microwaves | Internal heating of body tissue |
| Infrared | Skin burns |
| Ultraviolet | Skin cancer, cataracts (damages DNA) |
| X-rays | Ionising — damages DNA, cancer risk (limited exposure) |
| Gamma | Ionising — most damaging; causes cancer; kills cells |
Radio waves at normal intensity: no known significant health risk.
Refraction and the EM spectrum
When EM waves enter a different medium, they can:
- Reflect (bounce)
- Refract (change direction due to speed change)
- Absorb (energy transferred to medium)
- Transmit (pass through)
Different frequencies refract by different amounts — this is how a prism splits white light into a spectrum.
Ionising radiation from the EM spectrum
UV, X-rays and gamma rays are ionising — they can remove electrons from atoms, damaging DNA.
- Lower doses: increased cancer risk.
- Higher doses: radiation sickness; cell death.
✦Worked example— Worked example — wavelength to frequency
A radio station broadcasts at wavelength 3 m.
f = c/λ = (3×10⁸) / 3 = 10⁸ Hz = 100 MHz
Common exam errors
- Listing the EM spectrum in the wrong order — use the mnemonic.
- Confusing ionising (UV, X, γ) and non-ionising (radio, microwave, IR, visible) — only ionising damages DNA.
- Saying all EM waves are absorbed by the atmosphere — radio waves pass through; gamma/X-rays partially absorbed.
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