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.

    The energy of one photon of a particular frequency is:

    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

    WE WILL DISCUSS THESE TOPICS IN DETAIL LATER;
    FOR NOW, LET'S TURN TO:

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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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    TEMPERATURE SCALES

    Pretty much only the US has stuck with Farenheit temperatures;
    the rest of the world normally uses Celcius, but
    BECAUSE ENERGIES VANISH AT ABSOLUTE ZERO, THE NATURAL
    TEMPERATURE SCALE IS KELVIN and we'll usually use it.

  • The size of 1 degree C = 1 K and = 1.8 degrees F.
  • 0 C = 273.16 K (but we round it off)
  • The conversion formula is: F = (9/5)*C + 32
    or C = (5/9)*(F - 32)

  • -459 F = -273 C = 0 K is ABSOLUTE ZERO
  • -40 F = - 40 C = 233 K is when Farenheit and Celsius scales agree
  • 0 F = -18 C = 255 K
  • 32 F = 0 C = 273 K is freezing point of water
  • 68 F = 20 C = 293 K is room temperature
  • 99 F = 37 C = 310 K is body temperature
  • 212 F = 100 C = 373 K is boiling point of water
  • 10020 F = 5567 C = 5860 K is surface of Sun

  • BECAUSE ENERGIES VANISH AT ABSOLUTE ZERO, THE NATURAL
    TEMPERATURE SCALE IS KELVIN and we'll usually use it.

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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.

    Example: T = 5800 K = 5.8 x 10^3 K

  • lambda_{max} = 2.9 x 10^-1 cm.K / 5.8 x 10^3 K
  • = 0.5 x 10^-4 cm = 5 x 10^-5 cm = 500 nm = 5000 Angstroms

    WE DID SEVERAL OTHER EXAMPLES USING TODAY'S FORMULAS IN CLASS.
    SIMILAR EXAMPLES AND PROBLEMS ARE IN THE TEXTBOOK AND ON ITS WEBSITE.