Lecture 10: Lecture 10: Atomic Physics for Kids of All Ages

"If you aren't confused by quantum physics, then you really haven't understood it."

Niels Bohr

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    • Date: October 4, 1994
    Reading Assignment: pp. 91-105

    Description : line emission, absorption, emission

    Objectives

  • understand Kirchhoff's laws
  • understand the physical processes of emission
  • be able to describe the Bohr model of the atom
  • be able to describe how absorption is related to emission
  • understand the excitation processes
  • understand how temperature, motion, and atomic type can have on the spectra

    • Lecture Outline

    Slide # 1: Lecture 10: Atomic Physics for Kids of All Ages

  • Click here for subtitle
  • absorption and emission lines
  • KirchhoffÕs Laws
  • Bohr atom
  • formation of spectral lines
  • temperature, line broadening and molecules
    • Slide # 2: The Electromagnetic Spectrum
  • Radio
  • Infrared
  • Visible
  • Ultraviolet
  • X-ray
  • Gamma-ray
    • Slide # 3: Light: A Quick Review
  • Light has wave-like properties
  • Diffraction and Interference
  • Light has particle-like properties
  • Photoelectric Effect
    • Slide # 4: A Planck Curve
  • the signature of thermal emission
    • Slide # 5: The Planck Curve
  • Two laws describe the Planck Curve
  • WienÕs Law
  • StephanÕs Law
    • Slide # 6: The Doppler Effect
  • Motion can affect the wavelength and frequency of light
  • Motion does NOT change the speed of light
    • Slide # 7: Line Emission
  • Not all electromagnetic emission is thermal radiation
  • thermal radiation is one type of a continuous spectra
    • Slide # 8: Continuous and Line Spectra
  • Continuous Spectra have no spectral lines
    • Slide # 9: Spectral Lines
  • each element produces a unique set of spectral lines
  • analysis of the spectral lines can determine composition
    • Slide # 10: Spectral Lines
  • Emission lines
  • spectral lines which are bright
  • Absorption lines
  • spectral lines which are dark
  • found only in front of a continuous spectra
    • Slide # 11: Spectral Lines
  • Absorption lines are dark
  • Emission lines are bright
    • Slide # 12: A Spectral Line Experiment
  • Imagine a jar containing Hydrogen Gas
    • Slide # 13: Spectral Line Experiment
  • Add energy to the gas
  • Gas begins to glow
    • Slide # 14: Emission Line Spectra
  • bright lines
  • the observed spectra has emission lines
  • not continuous
    • Slide # 15: Spectral Line Experiment #2
  • Pass a bright continuum light through the jar
    • Slide # 16: A Spectral Line Experiment
  • An absorption spectra is observed on the screen
    • Slide # 17: Lines from the Gas
  • The dark lines are produced by the Hydrogen gas
  • Other gases produce other absorption lines
    • Slide # 18: Two Processes
  • Emission
  • Gas emits light
  • Energy is lost from the gas
  • Absorption
  • Gas absorbs light
  • Energy is absorbed by the gas
    • Slide # 19: Energy Absorbed
  • the H gas gains energy from the light
  • it emits this energy in all directions
    • Slide # 20: Emission and Absorption
  • both emission and absorption can occur from the same gas
    • Slide # 21: KirchhoffÕs Laws
  • describe the relationship between
  • emission lines
  • absorption lines
  • continuous spectra
    • Slide # 22: KirchhoffÕs 1st Law
  • a luminous solid, liquid or dense gas emits a continuous spectra
    • Slide # 23: KirchhoffÕs 2nd Law
  • a low-density, hot gas emits emission lines
  • the emission lines are characteristic of the composition of the gas
    • Slide # 24: KirchhoffÕs 3rd Law
  • a cool, low-density gas absorbs certain spectrum
  • the absorption lines are characteristic of the composition of the gas
    • Slide # 25: Gas density
  • High density
  • lots of atoms per cubic centimeter
  • thick
  • Low density
  • few atoms per cubic centimeter
  • thin or rarefied
    • Slide # 26: KirchhoffÕs Laws
  • Describes absorption, emission and continuous spectra
  • Does NOT explain the spectra
    • Slide # 27: Atomic Physics
  • line spectra depend on the type of elements in the gas
  • understanding atomic physics is ESSENTIAL to understanding the spectra
    • Slide # 28: The Bohr Model of the H- atom
  • proton is at the nucleus
  • protons have positive charge
  • electron orbits around the nucleus
  • electrons have negative charge
  • Electrical force holds the atom together
    • Slide # 29: The Bohr Model
  • the hydrogen atom
    • Slide # 30: Bohr Model
  • Electrons in the lowest energy state are in the ground state
  • Electrons with energies above a certain level are ionized
  • Electrons can only have certain discrete energy levels
    • Slide # 31: Electrons in the lowest energy state are in the ground state
  • no lower energy states exist
    • Slide # 32: Electrons with energies above a certain level are ionized
  • electron is no longer bound to the atom
    • Slide # 33: Electrons can only have certain discrete energy levels
  • discrete energy states
    • Slide # 34: Excited States
  • electrons which are not ionized and not in the ground states are in the excite
    • Slide # 35: Two Types of Excitation
  • Collisionally
  • atoms collide with other atoms
  • Radiatively
  • photons hit the atom
    • Slide # 36: Radiative Excitation
  • only photons of exactly the right energies can cause radiation excitation
  • photons of the wrong energy pass through the gas
    • Slide # 37: Radiative Excitation
  • photons of the wrong energy
    • Slide # 38: Radiative Excitation
  • photons of the right energy
    • Slide # 39: Radiative Excitation
  • photons of the right energy
    • Slide # 40: Transitions to Ground State
  • in 10-8 seconds, electron returns to the ground state
  • when electrons move to lower energy states, they radiate
  • the difference in the energy of the states is equal to the photon energy
    • Slide # 41: Line radiation
  • electron in ground state
    • Slide # 42: Line Radiation
  • collisional or radiative excitation
    • Slide # 43: Line Radiation
  • Electron drops to lower energy level
  • photon is emitted
    • Slide # 44: Line Emission
  • energy levels are at discrete energies
  • photons emitted have discrete energies
  • Photoelectric effect
  • photons emitted have random directions
    • Slide # 45: Absorption - step 1
  • photons absorbed from a continuous source
  • electron jumps to higher energy level
  • radiative excitation
    • Slide # 46: Absorption - step #2
  • electron spontaneous drops to lower energy state
  • emission
  • Emission is in a RANDOM direction
  • the continuous spectrum has dark lines
    • Slide # 47: Emission
  • the gas appears to have bright lines if there is no strong continuum source be
    • Slide # 48: Line Intensity
  • lines of a given element change intensity due to temperature
  • temperature determines the fraction of atoms at each excited level
  • temperature is related to collisional rate
  • collisional rate determines excitation rate
    • Slide # 49: Line Intensity
  • as the temperature goes up, most atoms will be in the 1st excited state
  • as the temperature goes up more, most atoms will be in 2nd excited state
    • Slide # 50: Line Intensity
  • temperature determines line intensity
    • Slide # 51: Line Broadening
  • motion changes the frequency of emitted light
  • atoms have random motion
    • Slide # 52: Line Broadening
  • random motions of gas make the line wider
    • Slide # 53: Types of Line Broadening
  • thermal
  • random gas motion
  • turbulent
  • churning of the gas
  • rotational
  • rotation of the gas
  • pressure
  • electrons overlapping
  • magnetic field
    • Slide # 54: Other types of Atoms
  • more electrons, protons and neutrons
  • electrical forces are different
  • energy levels are different
  • EVERY element has its own spectral lines
    • Slide # 55: Helium
  • 2 protons, 2 neutrons
  • 2 electrons
    • Slide # 56: Carbon
  • 6 neutrons, 6 protons
    • Slide # 57: Mixtures of Atoms
  • Each element emits its own spectral lines
  • The total spectrum is the sum of all spectral lines
    • Slide # 58: Modern View of the Atom
  • Orbits are not circular
  • electrons are found in a spherical ÒcloudsÓ around the nucleus
  • very similar to Bohr Model
    • Slide # 59: Molecules
  • Molecules have additional energy states
  • rotational
  • vibrational
  • Changes in rotational states produce photons
    • Slide # 60: Lecture 10: Atomic Physics for Kids of All Ages
  • absorption and emission lines
  • KirchhoffÕs Laws
  • Bohr atom
  • formation of spectral lines
  • temperature, line broadening and molecules