Quantum Physics

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Quantum Physics
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Quantum Theory

• Max Planck, examining heat radiation (ir light)
  proposes energy is quantized, or occurring in
  discrete small packets with a definite minimum
  value. (1901)
• All energy amounts were multiple of a certain
  constant, called Plancks constant h with a value
  of 6.63 x 10-34 Js
• Energy of radiation, E hf

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Max Planck 1858-1947
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The Photoelectric Effect

• Hertz observed spark discharges improved with
  illumination (1898)
• Illumination of metal by electromagnetic
  radiation causes emission of electrons
• Could not be explained by wave theory
• Frequency of e-m radiation determines if emission
  occurs, not intensity

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The Photoelectric Effect

• Each metal has minimum energy that must be
  supplied by light before emission occurs called
  the work function of the metal.
• Each metal found to have cutoff frequency of
  incident light below which no emission occurs, no
  matter how intense the light
• Only particle theory of light could explain this

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Einsteins Explanation

• Light consists of stream of massless particles
  called photons having energy hf, moving along
  electromagnetic waves (1905)
• When photon strikes electron, it gives up all its
  energy to the electron
• If hf gt w (work function of metal), emission
  occurs

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Einsteins Explanation

• Light intensity makes no difference if frequency
  is below a threshold frequency, ft
• Work function of metal h ft
• Max. electron kinetic energy hf - hft
• Einstein won Nobel Prize for explanation

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Albert Einstein 1879-1955
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Laws of Photoelectric Emission

• Rate of emission is directly proportional to
  intensity of incident light
• Kinetic energy of photoelectrons is independent
  of intensity of incident light.
• Maximum kinetic energy of photoelectrons varies
  directly with difference between frequency of
  incident light and cutoff frequency of metal

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Compton Shift

• Aurthur Compton sent X-rays into graphite
• X-rays scattered by collisions with electrons
  showed longer wavelength and lower energy
• Energy and momentum transferred from photon to
  electron
• Further support for particle nature of light

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The Compton Shift
Arthur Compton 1892-1962
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The Quantized Atom

• Rutherfords discovery of nucleus (1911) led to
  solar system model of atom
• Orbiting electrons contradicted e-m theory
• Niels Bohr (1913) proposed model of atom with
  electron orbits based on quantized energy states
• Difference between energy states always some
  multiple of Plancks constant

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Light Emission

• Electrons can absorb energy and jump to higher
  energy level
• When electrons fall to lower level, photon is
  emitted whose energy equals difference in the
  energy of the two levels
• Since frequency depends on energy, different
  energy changes cause different colors
  (frequencies) of light emitted

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Niels Bohr 1885-1962
Ernst Rutherford 1871-1937
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Line Spectra

• Unique structure of each element results in
  unique pattern of emission lines for each element
  allowing identification by spectroscopy
• When light passes through the element (usually as
  a low pressure gas) same frequencies are absorbed
  from the spectrum

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Continuous Spectrum

• When atoms are crowded together in a solid or
  dense gas, available energy levels are so
  numerous, all light frequencies are emitted
• White light is seen and when dispersed by a prism
  or diffraction grating, all colors are seen

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The Hydrogen Spectrum

• Hydrogen spectrum first to be analyzed
• Visible emission lines predicted and observed by
  Balmer called Balmer series.
• 6 uv emission lines (Lyman series) and 4 ir lines
  (Paschen series) discovered later.

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Matter and Waves

• de Broglie proposed wave-particle duality applied
  to matter particles, photons must have momentum
• E hf mc2 so mc h/l , photon momentum
• l h/mv , wavelength of particle with mass m and
  velocity v

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Louis de Broglie 1892-1987
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Matter and Waves

• All matter has wave properties but for large
  objects, wavelength is too small to be observed
• Wave nature of electron explains why only some
  orbits are stable standing wave must fit in
  orbit
• Whole number of wavelengths must equal
  circumference of orbit

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Matter Waves
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X-Ray Production

• Reverse of photoelectric effect
• High energy electron beam strike metal causing
  emission of photons
• EK hfmax - w
• Frequency of photon depends on speed of electron

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Werner Heisenberg 1901-1976
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The Uncertainty Principle (Heisenberg, 1927)

• It is impossible to simultaneously measure
  particles position and momentum accurately
• Measurement of one quantity changes the other
• Electrons location can only be described by
  probability

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Erwin Schrödinger 1887-1961
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The Wave Function

• Schrödinger (1926) proposed wave function (?)
  that describes subatomic particles
• Probability of electrons location can be found
  using the wave function
• Electron orbitals are probability distributions
  called electron clouds