THE NATURE OF LIGHT, PART 2

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.

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    HOW DO WAVES INTERACT WITH MATTER?

  • REFLECTED (angle of incidence equals angle of reflection)

  • TRANSMITTED (low opacity)

  • ABSORBED (high opacity)

  • REFRACTED (bent towards normal when entering a
    medium with a slower propagation speed)

  • INTERFERE (only a WAVE can do this)
    Either CONSTRUCTIVE (waves add when in phase)
    or DESTRUCTIVE (waves cancel when out of phase)

  • DIFFRACT (only a WAVE can do this)
    Waves spread out when passing through a hole or slit
    This is important only if the size of the hole or slit
    is comparable to the wavelength.

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    LIGHT AS PARTICLES

    ELECTROMAGNETIC ENERGY IS CARRIED BY PHOTONS:
    SINGLE QUANTA OF LIGHT.

    E = h nu = h c / lambda

    h = 6.63 x 10^{-34} J.s = 6.63 x 10^{-27} erg.s
    is PLANCK's CONSTANT.
    (Along with c, the speed of light; e, the charge on an
    electron (or proton) and G (Newton's constant of gravity),
    h is one of the FUNDAMENTAL CONSTANTS of NATURE.)

    These PHOTONS can equally well explain

  • REFLECTION,
  • REFRACTION,
  • TRANSMISSION and
  • ABSORPTION
    as can the Wave picture,

    BUT they can't explain

  • INTERFERENCE and
  • DIFFRACTION.

    On the other hand the WAVE picture can't explain:

  • The PHOTOELECTRIC EFFECT
  • and SPECTRAL LINES

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    PHOTOELECTRIC EFFECT

    Electrons can be expelled from many materials if light shines upon them.

    If the wavelength is TOO LONG (low frequency) nothing happens,
    EVEN IF the INTENSITY of the light is HIGH.

    Above a CRITICAL FREQUENCY the emitted electrons have a maximum
    energy (or velocity) that RISES with the FREQUENCY.

    E_e = h nu - h nu_{crit}

    Einstein pointed out that the wave theory could not explain this,
    while quanta of energy, with E = h nu could.

  • The wave theory predicted that even red light, if intense
    enough, would eject electrons -- but this never happened.
  • The wave theory also said that as the blue light was made brighter,
    faster electrons would emerge: instead only more of them came out, but their
    maximum kinetic (motion) energy was a function ONLY of the light's FREQUENCY.

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    SPECTRAL LINES

    By the mid 19th century chemists noticed specific
    colors of light coming from particular gases.

    Careful measurements indicated each element or
    compound produced a UNIQUE SET of EMISSION LINES:
    equivalent to FINGERPRINTS identifying the element.

    Spectra of the SUN and other STARS showed
    emission at most frequencies, but distinct dark bands,
    or ABSORPTION LINES, were also detected.

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    RADIATION, TEMPERATURE & POWER

    Crudely, hotter matter produces more highly accelerated charged
    particles, which therefore produces more powerful EM radiation.

    Heat energy is proportional to temperature:
    E = k T
    (where T is in Kelvins, 0 at ABSOLUTE ZERO).

    So the thermal (heat) energy in atoms should be
    proportional to the photon energy: using math

  • h nu propto k T OR
  • lambda propto 1/T

    (We use "propto" to mean "proportional to")

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    WIEN's LAW

    lambda_{max} = 0.29 cm.K / T (K)

    THIS IS THE PEAK WAVELENGTH FOR BLACKBODY
    (or THERMAL or PLANCKIAN) EMISSION FROM A
    SOLID, LIQUID or DENSE GAS.

    We will discuss this more in the next lecture.

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    Finish reading chapter 4.