Electromagnetic Waves

Electromagnetic (EM) waves are vibrations of electric and magnetic fields. They are transverse waves, so the vibrations are at right angles to the direction the wave travels. EM waves transfer energy without moving matter and do not need a medium (they can travel through space).

All EM waves travel at the same high speed in a vacuum: $$c = 3.0 \times 10^8\, \text{m s}^{-1}$$ This speed is approximately the same in air, but slower in glass and water.

The Electromagnetic Spectrum (low f/long λ → high f/short λ)

• Radio → Microwaves → Infrared → Visible light (red to violet) → Ultraviolet → X-rays → Gamma rays

As frequency increases, wavelength decreases. Higher frequency waves carry more energy.

Key Uses

• Radio: radio/TV broadcasting, astronomy, RFID
• Microwaves: satellite TV, mobile phones, microwave ovens
• Infrared: remote controls, grills, thermal imaging, intruder alarms, optical fibres
• Visible: human vision, photography, illumination
• Ultraviolet: security marking, checking bank notes, sterilising water
• X-rays: medical imaging, security scanners
• Gamma rays: sterilising food/equipment, detecting and treating cancers

Possible Dangers (with excessive exposure)

• Microwaves: internal heating of body cells
• Infrared: skin burns
• Ultraviolet: skin and eye damage; skin cancer risk
• X-rays and gamma: cell damage and mutations

Speed and Calculations

Wave speed equation: $v = f\,\lambda$. For EM waves in vacuum (or air), $v \approx c$.

How EM Waves Behave

• Reflection (mirrors, radio echoes)
• Refraction when speed changes (light bending in glass; lenses)
• Diffraction spreading when passing through gaps or around edges