# The Electromagnetic Spectrum — A-Level Physics
All electromagnetic (EM) waves are transverse waves consisting of oscillating electric and magnetic fields. They all travel at the speed of light in a vacuum and share common properties but differ in wavelength, frequency, and their interactions with matter.
1. Common Properties of All EM Waves
- Travel at m/s in vacuum
- Are transverse waves
- Can be polarised
- Transfer energy
- Can be reflected, refracted, and diffracted
- Follow
- Consist of oscillating E and B fields perpendicular to each other and to direction of travel
2. The Spectrum
| Region | Wavelength | Frequency | Sources | Detectors |
|---|---|---|---|---|
| Radio | >0.1 m | <3 GHz | Oscillating circuits | Aerial/antenna |
| Microwave | 1 mm – 0.1 m | 3 GHz – 300 GHz | Magnetron | Aerial |
| Infrared | 700 nm – 1 mm | 300 GHz – 430 THz | Hot objects | Thermometer, camera |
| Visible | 400–700 nm | 430–750 THz | Very hot objects, LEDs | Eye, CCD |
| Ultraviolet | 10–400 nm | 750 THz – 30 PHz | Sun, UV lamps | Fluorescent materials |
| X-rays | 0.01–10 nm | 30 PHz – 30 EHz | X-ray tube | Photographic film |
| Gamma | <0.01 nm | >30 EHz | Nuclear decay | GM tube |
Memory aid: Rogers Made Interesting Very Useful X-ray Gadgets
3. Uses
| Region | Uses |
|---|---|
| Radio | Communication, broadcasting, MRI |
| Microwave | Mobile phones, cooking, satellite communication, radar |
| Infrared | Heating, thermal imaging, remote controls, fibre optics |
| Visible | Vision, photography, fibre optics, lighting |
| UV | Sterilisation, fluorescent lamps, detecting forgeries |
| X-ray | Medical imaging, airport security, crystallography |
| Gamma | Cancer treatment, sterilisation, PET scans |
4. Hazards
| Region | Hazard | Mechanism |
|---|---|---|
| Microwave | Internal heating of body tissue | Absorbed by water molecules |
| IR | Skin burns | Absorbed, raises temperature |
| UV | Skin cancer, eye damage | Damages DNA, ionising |
| X-ray | Cell damage, cancer | Ionising radiation |
| Gamma | Cell damage, cancer | Strongly ionising |
Key Takeaways
EM waves form a continuous spectrum — boundaries are approximate
Higher frequency = higher photon energy ()
Ionising radiation (UV, X-ray, gamma) can damage cells by ionising atoms
Production: accelerating charges emit EM radiation
6. Practice Questions
- List three properties common to all EM waves. (3 marks)
- A radio wave has wavelength 1500 m. Calculate its frequency. (1 mark)
- Explain why gamma rays are more dangerous than radio waves. (2 marks)
- Give one use and one hazard for each: UV, X-rays, microwaves. (6 marks)
Answers
- Travel at in vacuum; transverse; can be polarised; can be reflected/refracted/diffracted; carry energy.
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Summary
- All EM waves: transverse, travel at , can be polarised
- Spectrum: Radio → Microwave → IR → Visible → UV → X-ray → Gamma
- Higher frequency = higher energy = more ionising
- Each region has specific uses and hazards