PROPERTIES OF LIGHT AS A WAVE

CONSIDER THE WAVE NATURE FIRST

Light is a TRANSVERSE EM WAVE.

Electric (E) fields oscillate perpendicular to Magnetic (B) fields
and the ENERGY FLOWS PERPENDICULAR to both fields.

Other transverse waves: SECONDARY (shear) SEISMIC waves.

LONGITUDINAL WAVES have ENERGY FLOWS PARALLEL to oscillations:
SOUND, PRIMARY (compressional) SEISMIC waves.

************************************************

CHARACTERISTICS OF ALL WAVES

  • FREQUENCY, nu (oscillations per s: Hertz -- Hz)
  • SPEED, V (depends on medium & perhaps, nu)
  • WAVELENGTH, lambda (distance between crests: meters)
  • KEY RELATION for all WAVES: lambda = V / nu
    or nu = V / lambda or lambda x nu = V
  • AMPLITUDE, A (strength of oscillation):
  • POWER proportional to A^2.

    ****************************************************

    FOR ELECTROMAGNETIC WAVES ONLY

  • V = c = 2.9979 x 10^8 m s^-1 = 3.00 x 10^10 cm s^-1
    = 3.00 x 10^5 km s^-1 = 186,000 miles per sec
  • THE SPEED OF LIGHT DOES NOT DEPEND UPON DIRECTION
    OR FREQUENCY IN A VACUUM: IT IS A CONSTANT OF NATURE.
  • In matter, V < c and the same frequency has a
    shorter wavelength than in vacuum.
  • The INDEX OF REFRACTION, n = c/V >= 1.
  • In air, n = 1.0003 , in normal glass, n ~= 1.5 .

    EM WAVES CAN ALSO BE POLARIZED: E field in a particular plane;
    B field in one perpendicular plane implies LINEAR POLARIZATION
    this is the only kind of polarization we'll worry about.

    ************************************************

    THE SEVEN BANDS OF THE ELECTROMAGNETIC SPECTRUM

    SEE THE FIGURE INSIDE THE FRONT COVER OF THE TEXTBOOK!

    RADIO: lambda > 1 cm, nu < 3 x 10^{10 Hz

  • Penetrates atmosphere if lambda < 15 m.
  • AM : nu ~ 1 MHz; FM: nu ~ 100 MHz
  • Therefore AM radio reflects off the ionosphere and can be heard
    at much greater distances than FM radio, which passes
    through the ionosphere.

    MILLIMETER (mm) or MICROWAVE : (1 cm > lambda > 3 x 10^{-3} cm;
    3 x 10^{10} Hz < nu < 1 x 10^{13} Hz.

  • Partially penetrates atmosphere --- molecules absorb.
  • This band is not shown in the book figure, but is usually noted.

    INFRARED (IR) : 3 x 10^{-3} cm > lambda > 7.2 x 10^{-5} cm = 720 nm,
    1 x 10^{13} Hz < nu < 4.2 x 10^{14} Hz

  • Partially penetrates atmosphere -- carbon dioxide, water, etc. absorb.
  • We feel IR as heat (these waves jiggle molecules in our skin).

    VISIBLE (OPTICAL) : 720 nm = 7200 A > lambda > 380 nm = 3800 A,
    4.2 x 10^{14} Hz < nu < 7.9 x 10^{14} Hz.

  • Penetrates atmosphere.
  • Visible spectrum: RED (longest wavelength), ORANGE, YELLOW,
    GREEN, BLUE, VIOLET (shortest wavelength -- highest frequency)
  • Our eyes evolved to see this light, since the Sun
    produces most of its radiation in this band,
  • and since nearly all of this radiation gets through the atmosphere.

    ULTRAVIOLET (UV) : 380 nm > lambda > 300 A,
    7.9 x 10^{14} Hz < nu < 1 x 10^{16} Hz

  • Mostly absorbed in atmosphere: ozone
  • A good thing, since UV radiation causes skin cancer.

    X-RAY : 300 A > lambda > 0.1 A,
    1 x 10^{16} Hz < nu < 3 x 10^{19} Hz

  • Absorbed in atmosphere: by any atom
  • A good thing too: X-rays can penetrate the
    body and cause cancer in many organs.

    GAMMA-RAY : lambda < 0.1 A, 3 x 10^{19} Hz < nu

  • The most energetic form of electromagnetic radiation.
  • Absorbed in atmosphere: by any atomic nucleus
  • A VERY good thing: gamma-rays quickly cause burns and cancer.

    ************************************************

    CHECK THAT THE PRODUCTS OF NU AND LAMBDA QUOTED
    FOR THE VARIOUS BANDS YIELD THE SPEED OF LIGHT SO AS TO
    UNDERSTAND THE DIFFERENT UNITS OF LENGTH WE USE.

    *************************************************************

    HOW ARE ElectroMagnetic WAVES MADE?

    Most come from ATOMIC, MOLECULAR OR NUCLEAR TRANSITIONS.

    I.e., electrons or protons changing quantum states.

    BUT FUNDAMENTALLY, EM RADIATION IS PRODUCED BY
    AN ACCELERATED CHARGED PARTICLE.

    Since ELECTRONS have the LOWEST MASSES they are
    MOST EASILY ACCELERATED, therefore, yield most EM waves.

    SYNCHROTRON RADIATION: produced by electrons spiralling
    around magnetic field lines, when moving at v ~= c.

  • The circular part of the motion is ACCELERATED
    and produces the radiation
  • Any motion along the direction of the B field is at
    a constant speed and does not make radiation.
  • Synchrotron radiation is strongly POLARIZED; most
    other EM radiation we will talk about is basically UNPOLARIZED.

    ***************************************************************