The Nature of Waves

Summary

• Definition
  • A wave is a disturbance that propagates through a medium.
    • verb, propagate; noun, propagation: refers to the transmission of a disturbance from one location to another.
    • singular, medium; plural, media: refers to the intervening substance(s) through which a disturbance is transmitted.
• Essential property
  • Waves transfer energy, momentum, and information, but not mass.
• Classifying waves by medium
  • Mechanical waves
    • Matter is the medium.
    • Sound is a mechanical wave.
  • Electromagnetic waves
    • Electric and magnetic fields are the media.
    • Light is an electromagnetic wave.
    • Electromagnetic waves include, radio waves, microwaves, infrared, light, ultraviolet, x-rays, and gamma rays.
    • Electromagnetic waves are dealt with more fully in another section of this book.
  • Gravitational waves
    • The gravitational field is the medium.
  • Matter waves
    • The quantum mechanical description of fundamental particles like electrons and quarks as a wave
• Classifying waves by type of disturbance
  • Transverse waves
    • The disturbance is perpendicular to the direction of propagation.
    • All electromagnetic waves are transverse. This includes light.
    • A crest is a point of maximal change in the positive direction.
    • A trough is a point of maximal change in the negative direction.

    Animate

    Magnify

  • Longitudinal waves
    • The disturbance is parallel to the direction of propagation.
    • Sound is a longitudinal wave.
    • A compression or condensation is a region where the medium is under compression.
    • A rarefaction or dilation is a region where the medium is under tension.

    Animate

    Magnify

  • Complex waves or surface waves
    • A combination transverse-longitudinal wave that forms near the surface of some media
    • Waves on water (deep water waves or tiny ripples) are complex waves.

    Animate

  • Torsional waves
    • The disturbance causes the medium to twist.
    • Are significant in some large structures like bridges and skyscrapers.

    Animate

• Classifying waves by duration
  • adjective: episodic, intermittent, sporadic; noun: pulse, burst, surge, spike, transient
    • The disturbance is momentary and sudden.
  • adjective: periodic, harmonic, oscillatory; noun: wave train
    • The disturbance repeats continuously and at regular intervals.
    • The next section in this book is all about periodic waves.
• Classifying waves by appearance
  • Traveling waves…
    • appear to move.
  • Standing waves or stationary waves…
    • do not appear to move.
    • are dealt with in a separate section of this book.

  A traveling wave in action

  A standing wave in action