The Atom, Light Emission, and the Quantum

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The Atom, Light Emission, and the Quantum
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Models

• Impossible to know what atoms look like since
  theyre so small.
• Instead, use models (conceptual, graphical and or
  mathematical representation)

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Classic model of the atom

• Planetary model. Very outdated, and not an
  accurate representation of the atom.
• Still useful for understanding certain processes,
  like light emission

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Light Quanta

• Is light a particle or wave?
• Wave behaviors of light include reflection,
  refraction, diffraction, and interference
• Light also displays particle behavior (example
  later)

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Light Quanta

• Light occurs in elemental quantized packets of
  energy, called photons. A quantum is a unit--the
  smallest amount of something
• Think-The mass of a gold ring is the mass of a
  single gold atom multiplied by the number of
  atoms in the ring.

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• The energy of the emitted photon is related to
  the frequency, given by the equation E hf
  (proposed by Max Planck)
• h Plancks constant (6.6 x 10-34 Js)
• Constant in nature of Energy to frequency
• Sets a limit on the smallness of things
• Gives the smallest amount of energy that can be
  converted to light with frequency f
• So light is emitted from an atom as a stream of
  photons, each photon with a frequency and energy
  of hf

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Photoelectric Effect

• Einstein found support for quantum theory of
  light in the Photoelectric Effect
• The ejection of electrons from certain metals
  when light falls upon them. (these metals are
  photosensitive)
• High frequency light, even when dim, can eject
  electrons from a photosensitive surface
• Low frequency light, even if very bright, may not
  be able to eject electrons from the surface

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Photoelectric Effect

• Einstein explained by thinking of light in terms
  of photons, instead of continuous waves
• The number of photons in a beam controls the
  brightness. The frequency of the light controls
  the energy of individual photons

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Photoelectric Effect
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Waves as particles

• So what is light? A particle? A wave? A particle
  that waves as it goes by?
• Light shows properties of both
• This is known as Wave-Particle Duality

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Particles as Waves

• Recall Thomas Youngs double slit experiment that
  showed an interference pattern

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• Shoot beams of photons (dim light)
• An interference pattern forms photon by photon on
  the screen
• Each single photon has wave and particle
  properties. Different aspects show at different
  times
• A photon behaves as a particle when it is being
  emitted or absorbed by detectors, and behaves as
  a wave in traveling to a source to a detector

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Particles as waves

• So if light can show wave and particle
  properties, what about other forms of matter?
• Turns out that all bits of matter have wave
  properties
• Explained by Louis de Broglie
• Wavelength h/momentum

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Check your understanding

• Does a 0.5 kg baseball moving 10 m/s have a
  wavelength?

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Electron waves

• You usually think of electrons as negatively
  charged particles
• In Bohring Spectra, you saw that electrons have
  specific energy levels.
• This is best understood by considering the wave
  properties of an electron

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Orbital radius of electron

• Whole number integers of de Broglie wavelengths
  are needed for an orbital radius to be possible.
• Will the above radius work?

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How about this one?
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De Broglie Wavelengths
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Black Body Radiation
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A Particle Model of Waves
Section
27.1
Radiation from Incandescent Bodies

• When the dimmer control is used to increase the
  voltage to the bulb, the temperature of the
  glowing filament increases.
• As a result, the color changes from deep red to
  orange to yellow and finally, to white.

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A Particle Model of Waves
Section
27.1
Radiation from Incandescent Bodies

• This color change occurs because the
  higher-temperature filament emits
  higher-frequency radiation.
• The higher-frequency radiation comes from the
  higher-frequency end of the visible spectrum (the
  violet end) and results in the filament appearing
  to be whiter.

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A Particle Model of Waves
Section
27.1
Radiation from Incandescent Bodies

• What would you expect to see if you viewed the
  glowing filament through a diffraction grating?
• When viewed in this way, all of the colors of the
  rainbow would be visible.
• The bulb also emits infrared radiation that you
  would not see.
• A plot of the intensity of the light emitted from
  a hot body over a range of frequencies is known
  as an emission spectrum.

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A Particle Model of Waves
Section
27.1
Radiation from Incandescent Bodies

• Emission spectra of the incandescent body at
  temperatures of 4000 K, 5800 K, and 8000 K are
  shown in the figure. Note that at each
  temperature, there is a frequency at which the
  maximum amount of energy is emitted.

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The Bohr Model of the Atom
Quantized Energy

• As shown in the figure below, the quantization of
  energy in atoms can be likened to a flight of
  stairs with decreasing-height steps.
• To go up the stairs, you must move from one step
  to the nextit is impossible to stop at a
  midpoint between steps.

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The Bohr Model of the Atom
Energy of an Atom

• The change in energy of the atom equals the
  energy of the emitted photon.

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The Bohr Model of the Atom
Energy and Electron Transitions

• Some of hydrogens energy levels and the possible
  energy level transitions that it can undergo are
  shown in the figure at right.
• Note that an excited hydrogen atom can emit
  electromagnetic energy in the infrared, visible,
  or ultraviolet range depending on the transition
  that occurs.

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The Bohr Model of the Atom
Energy and Electron Transitions

• Ultraviolet light is emitted when the atom drops
  into its ground state from any excited state.
• The four visible lines in the hydrogen spectrum
  are produced when the atom drops from the n 3
  or higher energy state into the n 2 energy
  state.

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Light Emission Spectra
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Atoms and Light Emission