Lecture 3: Spectroscopy-Jumpy Electrons and Fleeting Photons

"You have to wait for the right one to come along..."

Mom

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    • Date: January 31, 1995
    Reading Assignment: pp. 79-100

    Description : Doppler Shifts and Atomic Physics

    Objectives

  • be able to describe what the Doppler shift is and why it occurs
  • be able to describe when red shifts occur and when blue shifts occur
  • be able to distinguish between ground and excited energy states of electrons
  • be able to describe when photons are emitted from atoms
  • be able to describe the two ways electrons the two main ways electrons are excited
  • be able to list and apply Kirchoff's laws

    • Lecture Outline

    Slide # 1: Spectroscopy

  • Jumpy Electrons and Fleeting Photons
  • Nailing the Planck Curve
    • Slide # 2: Red Light vs Blue Light
  • Red has a longer wavelength
  • Red has a lower frequency
  • Remember: Longer wavelengths = lower frequencies
  • Red photons have lower energy than Blue photons
    • Slide # 3: Atmospheric Opacity
  • some wavelengths are blocked by the atmosphere
    • Slide # 4: Electromagnetic Spectra
  • radio
  • infrared
  • optical
  • ultraviolet
  • x-ray
  • gamma-ray
    • Slide # 5: Thermal Radiation
  • Intensity is only related to temperature
  • Independent of the type of material
    • Slide # 6: The Planck Curve (GRAPHICS)
  • The position of the peak and the area under the curve depend on Temperature
    • Slide # 7: Wien's Law (GRAPHICS)
  • Peak Wavelength in cm
  • T in Kelvin
    • Slide # 8: Stephan's Law (GRAPHICS)
  • sigma is
  • Stephan-Boltzmann constant
  • 5.67 x 10-5 erg/s/cm2/K4
    • Slide # 9: Thermal Radiation vs Photoelectric Effect
  • Stephan's and Wien's laws are for thermal radiators
  • the Photoelectric effect applies to individual photons
    • Slide # 10: Light (GRAPHICS)
  • EM Radiation
    • Slide # 11: EM Radiation (GRAPHICS)
  • full spectra
    • Slide # 12: Thermal Radiation (GRAPHICS)
  • the Planck Curve
    • Slide # 13: The Doppler Effect
  • Motion can affect the wavelength and frequency of light
  • Motion does NOT change the speed of light
    • Slide # 14: Motion and Wavelength (GRAPHICS)
  • Imagine traveling toward a light emitting source
    • Slide # 15: Motion and Wavelength (GRAPHICS)
  • Observer #2 encounters more peaks per second than observer #1 due to his motio
    • Slide # 16: Doppler
  • Blue Shift
  • higher frequencies, shorter wavelength
  • the observer moves toward the source
  • the source moves toward the observer
  • Red Shift
  • lower frequencies, longer wavelengths
  • the observer moves away from the source
  • the source moves away from the observer
    • Slide # 17: Doppler Effect
  • the change in wavelength is proportional to velocity
    • Slide # 18: Doppler Effect
  • the ratio of apparent and true wavelength depends of velocity
    • Slide # 19: Constellation Corner (GRAPHICS)
  • The Constellation de Jour
    • Slide # 20: Draco (GRAPHICS)
  • Fairfax - Feb 1 - 10pm - NNE - 6.0
    • Slide # 21: Draco (GRAPHICS)
  • Fairfax - Feb 1 - 10pm - NNE - 6.0
    • Slide # 22: Draco (GRAPHICS)
  • Fairfax - Feb 1 - 10pm - NNE - 5.0
    • Slide # 23: Draco (GRAPHICS)
  • Fairfax - Feb 1 - 10pm - NNE - 4.0
    • Slide # 24: Draco (GRAPHICS)
  • Fairfax - Feb 1 - 10pm - NE
    • Slide # 25: Continuous and Line Spectra
  • Continuous Spectra have no spectral lines
    • Slide # 26: Spectral Lines
  • each element produces a unique set of spectral lines
  • analysis of the spectral lines can determine composition
    • Slide # 27: 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 # 28: Spectral Lines (GRAPHICS)
  • Absorption lines are dark
  • Emission lines are bright
    • Slide # 29: A Spectral Line Experiment (GRAPHICS)
  • Imagine a jar containing Hydrogen Gas
    • Slide # 30: Spectral Line Experiment #1 (GRAPHICS)
  • Add energy to the gas
  • Gas begins to glow
    • Slide # 31: Emission Line Spectra (GRAPHICS)
  • bright lines
  • the observed spectra has emission lines
  • not continuous
    • Slide # 32: Spectral Line Experiment #2 (GRAPHICS)
  • Pass a bright continuum light through the jar
    • Slide # 33: A Spectral Line Experiment (GRAPHICS)
  • An absorption spectra is observed on the screen
    • Slide # 34: Lines from the Gas
  • The dark lines are produced by the Hydrogen gas
  • Other gases produce other absorption lines
    • Slide # 35: Two Processes
  • Emission
  • Gas emits light
  • Energy is lost from the gas
  • Absorption
  • Gas absorbs light
  • Energy is absorbed by the gas
    • Slide # 36: Energy Absorbed (GRAPHICS)
  • the H gas gains energy from the light
  • it emits this energy in all directions
    • Slide # 37: Emission and Absorption (GRAPHICS)
  • both emission and absorption can occur from the same gas
    • Slide # 38: Kirchhoff's Laws
  • describe the relationship between
  • emission lines
  • absorption lines
  • continuous spectra
    • Slide # 39: Kirchhoff's 1st Law
  • a luminous solid, liquid or dense gas emits a continuous spectra
    • Slide # 40: Kirchhoff's 2nd Law
  • a low-density, hot gas emits emission lines
  • the emission lines are characteristic of the composition of the gas
    • Slide # 41: Kirchhoff's 3rd Law
  • a cool, low-density gas absorbs certain spectrum
  • the absorption lines are characteristic of the composition of the gas
    • Slide # 42: Gas density
  • High density
  • lots of atoms per cubic centimeter
  • thick
  • Low density
  • few atoms per cubic centimeter
  • thin or rarefied
    • Slide # 43: Kirchhoff's Laws
  • Describes absorption, emission and continuous spectra
  • Does NOT explain the spectra
    • Slide # 44: Atomic Physics
  • line spectra depend on the type of elements in the gas
  • understanding atomic physics is ESSENTIAL to understanding the spectra
    • Slide # 45: The Bohr Model (GRAPHICS)
  • the hydrogen atom
    • Slide # 46: 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 # 47: Electrons can only have certain discrete energy levels (GRAPHICS)
  • discrete energy states
    • Slide # 48: Excited States
  • electrons which are not ionized and not in the ground states are in the excite
    • Slide # 49: Two Types of Excitation
  • Collisionally
  • atoms collide with other atoms
  • Radiatively
  • photons hit the atom
    • Slide # 50: Radiative Excitation
  • only photons of exactly the right energies can cause radiation excitation
  • photons of the wrong energy pass through the gas
    • Slide # 51: Radiative Excitation (GRAPHICS)
  • photons of the wrong energy
    • Slide # 52: Radiative Excitation (GRAPHICS)
  • photons of the right energy
    • Slide # 53: 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 # 54: Line radiation
  • electron in ground state
    • Slide # 55: Line Radiation (GRAPHICS)
  • collisional or radiative excitation
    • Slide # 56: Line Radiation (GRAPHICS)
  • Electron drops to lower energy level
  • photon is emitted
    • Slide # 57: Emission and Absorption (GRAPHICS)
  • both emission and absorption can occur from the same gas
    • Slide # 58: 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 # 59: 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 # 60: Line Intensity
  • temperature determines line intensity