Wave-Particle Duality and Quantum Theory by qfa20129


									Wave-Particle Duality and
   Quantum Theory
I. Light as a Wave
     Huygens’ Principle (1670)
• Every point on any wave
  front can be regarded as
  a new point source of
  secondary waves.
• Plane waves diffract and
  become circular waves.
• The smaller the opening
  is compared to the
  wavelength, the greater
  the bending of the waves.
  Young’s Interference Experiment
• When monochromatic
  light passes through a
  double slit, the bright
  spots show
  interference and the
  dark spots show
II. Light as a Particle
         Max Planck (1900)
• Light is “quantized”- it is absorbed and
  emitted in chunks of energy called
• The energy of a photon is directly
  proportional to its frequency.
• E = hf
       Albert Einstein (1905)
• Photoelectric effect: electrons are ejected
  from certain metals by light acting as a
  stream of photons
• The intensity (brightness) of light does not
  matter if the frequency of light is too low
  and it does not have enough energy to
  eject electrons.
          Niels Bohr (1913)
• The Bohr model of the atom explained
  why elements only emit certain
  frequencies of light and supported the
  theory that light is quantized.
             Bohr Atomic Model
• Electrons move at different distances from the nucleus
  according to the amount of energy they have.

• An electron can absorb energy and move to a higher
  energy level. It is said to be excited.

• The electron is unstable in the higher energy level.

• It will lose the extra energy and fall back to its original

• As the electron falls back, a photon of light is emitted!
Absorption and Emission
Sodium (Na)
Neon (Ne)
Mercury (Hg)
Helium (He)
  III. If a wave can act like a
particle, can a particle act like a
      Louis De Broglie (1924)
• All particles have a wavelength related to
  the momentum of the particles:

            Wavelength = h/mv

• Electrons act as waves, and their
  wavelengths must fit evenly into the
  circumferences of their orbits.
• Calculate your de Broglie wavelength if
  you have a mass of 60 kg and a velocity of
  2 m/s. (h = 6.6 x 10^-34 Js)
• 5.5 x 10^-36 m
• Calculate the de Broglie wavelength of an
  electron with a mass of 9.1x10^-31 kg and
  a velocity of 6000000 m/s.
• 1.2 x 10^-10 m
Schrodinger’s Wave Equation
       Electron Microscopes
• Electron microscope: The wavelength of a
  beam of electrons is thousands of times
  shorter than that of visible light, so
  electron microscopes can detect more

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