5.7 - Electromagnetic Waves

Electromagnetic Fields and Early Discoveries

• Electromagnetic waves consist of oscillating electric and magnetic fields perpendicular to each other. Their oscillations are also perpendicular to the direction of propagation.
• 1801 — Thomas Young: demonstrated the wave nature of light using interference.
• 1862 — James Maxwell: proposed that light is an electromagnetic wave.
• 1887 — Heinrich Hertz: generated radio waves, another type of electromagnetic wave.
• By the early twentieth century, the electromagnetic spectrum included gamma rays, X-rays, ultraviolet rays, visible light, infrared rays, microwaves and radio waves.

Characteristics

• Electromagnetic waves are transverse waves.
• They do not require a medium and can propagate through a vacuum.
• All travel in a vacuum at:

• Their speed is lower in a material medium.
• Under suitable conditions, they exhibit reflection, refraction, diffraction and interference.

Electromagnetic Spectrum

• The electromagnetic spectrum is continuous: there are no definite boundaries separating adjacent wave types.
• In order of increasing wavelength:
  1. Gamma rays
  2. X-rays
  3. Ultraviolet rays
  4. Visible light
  5. Infrared rays
  6. Microwaves
  7. Radio waves
• Frequency increases in the reverse order. For a fixed speed, greater wavelength corresponds to lower frequency.
• The illustrated visible spectrum extends approximately from $400\text{ nm}$ to $750\text{ nm}$, from violet towards red.
• Energy carried by electromagnetic waves is directly proportional to frequency. Gamma rays and X-rays carry large amounts of energy and require proper handling to avoid harm.

Applications

Wave	Applications
Radio waves	Long-distance radio communication; local radio and television broadcasting; wireless communication such as Bluetooth, Wi-Fi, Zigbee and Z-Wave; millimetre-wave body scanners at airports.
Microwaves	Satellite communication; mobile phone networks; communication between electronic devices; aircraft radar and speed traps; microwave ovens.
Infrared rays	Ovens, grills and toasters; infrared cameras and binoculars for night vision; drying car paint; treatment of muscle pain; television and DVD remote controls.
Visible light	Vision; photography; photosynthesis; laser applications in cutting metal, surveying land and transmitting information through optical fibres.
Ultraviolet rays	Hardening dental fillings; checking currency notes; purifying drinking water; sterilising medical instruments and food; insect traps.
X-rays	Detecting fractures and examining internal organs; checking welded joints; scanning baggage; checking the authenticity of paintings.
Gamma rays	Killing cancer cells in radiotherapy; sterilising surgical and medical equipment in bulk; food irradiation to extend storage life.