Waves & The Electromagnetic Spectrum

What a wave is

• A wave is a disturbance produced by an oscillating source that transfers energy (and information) from place to place without transferring matter
• The particles of the medium oscillate about a fixed position; the wave moves through them, but they do not travel with it
• A toy duck floating on water is the classic visualisation: a passing ripple lifts the duck up and lets it back down, but the duck itself stays roughly where it was

Two ways a wave can vibrate

• All waves fall into one of two categories, set by which direction the medium vibrates relative to the wave's travel:
  • Transverse waves: the oscillation is perpendicular to the direction of energy transfer
  • Longitudinal waves: the oscillation is parallel to the direction of energy transfer

Transverse waves

• A transverse wave shows alternating peaks (the highest points above the rest position) and troughs (the lowest points below it)
• Transverse waves can travel through solids and along the surface of a liquid, but a mechanical transverse wave cannot travel through the body of a liquid or any gas (the molecules have no lateral spring to vibrate against)
• Electromagnetic waves are transverse and can travel through solids, liquids, gases, and a vacuum; they are the only transverse waves not bound by needing a material medium
• Everyday examples of transverse waves:
  • ripples on the surface of a pond
  • a flicked rope or skipping line
  • the vibrations of a plucked guitar string
  • S-waves in earthquakes (one of the two main seismic wave types)
  • all electromagnetic waves (light, radio, X-rays, microwaves and so on)

Longitudinal waves

• A longitudinal wave shows alternating compressions (particles bunched closer together than average) and rarefactions (particles spread further apart than average)
• The medium's density and pressure rise and fall as the wave passes
• Longitudinal waves travel through solids, liquids and gases because the medium can be compressed and expanded along the line of travel; they cannot travel through a vacuum because there are no particles to compress
• Everyday examples of longitudinal waves:
  • sound waves in air, water and steel
  • P-waves in earthquakes (the faster of the two main seismic wave types)
  • pressure waves in any fluid (a sudden push on a piston of water)

Side-by-side comparison