The electromagnetic spectrum · Grade 7

Light is one member of a family of electromagnetic waves. In order of increasing frequency they are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. All travel at 3.0 × 10⁸ m/s in a vacuum. Each has uses – from phones to cancer treatment – and the higher-frequency ones can be dangerous.

Key facts

  • Order (low to high frequency): radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Wavelength goes the other way.
  • All EM waves are transverse and travel at c = 3.0 × 10⁸ m/s in a vacuum: c = fλ.
  • Uses: radio – broadcasting; microwaves – cooking, phones, satellites; infrared – remotes, thermal cameras; visible – sight, optical fibres; UV – sterilising, security marks; X-rays – imaging bones; gamma – sterilising, radiotherapy.
  • UV, X-rays and gamma rays are ionising: they can damage cells and DNA.
  • Photon energy is proportional to frequency (E = hf).
  • Radar and signal timing: distance = c × time (÷ 2 for an echo).

Worked examples

Example 1

A mobile phone uses microwaves of frequency 1.8 GHz. Find the wavelength.

Answer: 0.167 m (16.7 cm)

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