PHYSICS em waves

• Electromagnetic Waves


  Mechanical waves that can propagate through empty space
• Maxwell's equations form the theoretical basis of all electromagnetic waves that propagate through space
• Hertz confirmed Maxwell's prediction when he generated and detected electromagnetic waves in 1887
• Electromagnetic waves


  Generated by oscillating electric charges, and the waves radiated from the oscillating charges can be detected at great distances
• Electromagnetic waves


  Carry energy and momentum
• Hertz's experiment


  • An induction coil is connected to a transmitter with two spherical electrodes separated by a gap
  • The discharge between the electrodes exhibits an oscillatory behavior at a very high frequency
  • Sparks were induced across the gap of the receiving electrodes when the frequency of the receiver was adjusted to match that of the transmitter
• Linearly polarized waves


  Waves in which the electric and magnetic fields are parallel to a pair of perpendicular axes
• Plane electromagnetic waves


  Waves that travel in space, where the magnitudes of the electric and magnetic fields depend upon position and time only
• Rays


  Lines along which the wave travels
• Plane wave


  The collection of waves
• Spherical wave


  The surface connecting points of equal phase is a sphere
• Momentum


  Electromagnetic waves transport momentum as well as energy
• Dipole antenna


  Two conducting rods connected to a source of alternating voltage, where the length of each rod is one-quarter of the wavelength of the radiation to be emitted
• Angular dependence of intensity


  The intensity of the radiation varies as (sin² θ) / r², where θ is the angle from the antenna and r is the distance from the origin
• Whenever a charged particle accelerates, it radiates energy
• Types of electromagnetic waves


  • Infrared waves
  • Visible light
  • Ultraviolet light
  • X-rays
  • Gamma rays
  • Radio waves
  • Microwaves
• Visible light is a small portion of the electromagnetic spectrum
• Types of electromagnetic waves are distinguished by frequency or wavelength
• Looking at objects in different portions of the electromagnetic spectrum can produce different information
• Radio Waves


  • Wavelengths of more than 104 m to about 0.1 m
  • Used in radio and television communication systems
• Microwaves


  • Wavelengths from about 0.3 m to 10-4 m
  • Well suited for radar systems
  • Microwave ovens are an application
• Infrared Waves


  • Wavelengths of about 10-3 m to 7 x 10-7 m
  • Incorrectly called "heat waves"
  • Produced by hot objects and molecules
  • Readily absorbed by most materials
• Visible Light


  • Part of the spectrum detected by the human eye
  • Most sensitive at about 5.5 x 10-7 m (yellow-green)
• Ultraviolet Light


  • Covers about 4 x 10-7 m to 6 x 10-10 m
  • Sun is an important source of uv light
  • Most uv light from the sun is absorbed in the stratosphere by ozone layer
• Xrays


  • Wavelengths of about 10-8 m to 10-12 m
  • Most common source is acceleration of high-energy electrons striking a metal target
  • Used as a diagnostic tool in medicine
• Gamma Rays

  • Wavelengths of about 10-10 m to 10-14 m
  • Emitted by radioactive nuclei
  • Highly penetrating and cause serious damage when absorbed by living tissue
• Looking at objects in different portions of the spectrum can produce different information
• Images of the Crab Nebula

  • X-rays
  • Visible light
  • Radio waves
  • Infrared waves