The Interaction of Light and Matter

• Spectral Lines• Photons• Rutherford-Bohr Model of the

Atom• Quantum Mechanics and

Wave-Particle Duality

How can we know about the stars…?

• French Philosopher Auguste Comte (1798-1857) stated that it would not be possible to know the chemical or mineralogical nature of the stars…

•Knowledge of the stars and other astrophysical objects relies on the information that is provided to us through electromagnetic radiation….•To do astronomy one must know physics…Need to understand light and how it interacts with matter. Fortunately we understand a lot…

Fraunhofer Spectral Lines

• Josef von Fraunhofer(1787-1826) had catalogued 475 dark spectral lines in the solar spectrum.

• Fraunhofer showed that we can learn the chemical composition of the stars. Identified a spectral line of sodium in the spectrum of the Sun

Spectral LinesKirchoff’s Laws of Spectra

• Robert Bunsen (1811-1899) created a burner that produced a “colorless” flame ideally suited for studying the spectra of heated substances.

• Gustav Kirchoff (1824-1887) and Bunsen designed a spectroscope that could analyze the emitted light.

• Kirchoff determined that 70 of the dark lines in the solar spectrum corresponded to the 70 bright lines emitted by iron vapor.

• Chemical Analysis by Spectral Observations “Spectral Fingerprint”.

•A hot dense gas or hot solid object produces a continuous spectrum with no dark spectral lines

•A hot, diffuse gas produces bright spectral lines (emission lines)

•A cool, diffuse gas in front of a source of a continuous spectrum produces dark spectral lines (absorption lines) in the continuous spectrum.

Spectral Lines(1868) New element discovered in Sun!!

• Helium discovered in solar spectrum by Pierre Janssen in 1868

Spectral LinesApplication of Spectral Measurements

• Stellar Doppler Shift• Galactic Doppler Shifts• Quasar Doppler Shifts

Spectral LinesSpectrographs

• Spectroscopy• Diffraction grating equation

(n=0,1,2,…)

• Resolving Power

PhotonsPhotoelectric Effect

• Photoelectric Effect• Kinetic energy of ejected

electrons does not depend on intensity of light!

• Increasing intensity will produce more ejected electrons.

• Maximum kinetic energy of ejected electrons depends on frequency of light.

• Frequency must exceed cutoff frequency before any electrons are ejected

Einstein took Planck’s assumption of quantized energy of EM waves seriously. Light consisted of massless photons whose energy was:

Einstein was awarded the Nobel Prize in 1921 for his work on the photo-electric effect

PhotonsCompton Scattering

• Compton Scattering• Wikipedia entry• Arthur Holly Compton (1892-

1962) provided convincing evidence that light manifests particle-like properties in its interaction with matter by considering how (x-ray) photons can “collide” with a free electron at rest.

• Conservation of energy and momentum leads to the following:

• Showed that photons are massless yet carry momentum!!!

Compton Wavelength:

is the characteristic change in wavelength in the scattered photon.

What is matter?

Let’s Shoot stuff at it to find out…

Rutherford-Bohr Model of the Atom

• Ernest Rutherford (1871-1937)

• Rutherford Scattering

Observation consistent with scattering from a very small (10,000 times smaller radius than the atom) dense object… The nucleus

Models of the Atom

• Greek Model of the Atom. Democritus, Leucippus et al

• “Plum Pudding” model of the atom. J.J. Thomson et al.

• Classical solar system model.

• Bohr Model of the atom.

• De Broglie Model of the atom.

• Schroedinger Model of the Atom (english)

• http://phet.colorado.edu/sims/hydrogen-atom/hydrogen-atom.jnlp

Wavelengths of Hydrogen

Bohr’s Semi-classical Model of the Atom

• Bohr assumed for the electron proton system – to be subject to Coulomb’s

Law for electric charges

– Quantization of angular momentum for the electron orbit. L=nh/2

• Quantization condition prevents electron from continuosly radiating away energy

• Discrete energy levels for electron orbit

Bohr’s Semi-classical Model of the Atom

Bohr’s Semi-classical Model of the Atom

Bohr’s Semi-classical Model of the Atom

Bohr’s Semi-classical Model of the Atom

Bohr’s Semi-classical Model of the Atom

Bohr Hydrogen atom and spectral lines

Quantum Mechanics and Wave-Particle Duality

• QM Tunneling• DeBroglie Matter

Wave• Heisenberg

Uncertainty Principle• Schroedinger

Equation and QM atom

• Spin and Pauli Exclusion Principle

DeBroglie Matter Wave

Quantum Mechanics and Wave-Particle Duality

• Fourier • Wave Packets

Uncertainty Principle

Uncertainty Principle

Uncertainty Principle

Schrodinger Wave Equation and Hydrogen Atom

Additional Quantum Numbers and splitting of spectral lines

• Normal Zeeman Effect

Can measure magnetic fields by examining spectra!!!!

Spin and the Pauli Exclusion Principle

No two electrons can share the same set of four quantum numbers

Chemistry….The periodic table….

Electron Degeneracy Pressure and White Dwarfs

• Exclusion Principle for Fermions (spin-1/2 particles) and uncertainty principle provide electron degeneracy pressure that is the mechanism that prevents the further collapse of white dwarves….

The Complex Spectra of Atoms

• Number of possible energy levels increases with number of electrons. n,l,m,…

• Interactions with external magnetic fields, etc…

Complicated Spectra!!!!

Links

• http://sci2.esa.int/interactive/media/flashes/2_2_1.htm• http://wps.prenhall.com/wps/media/objects/610/625137/Chaisson/CH.00.00

2/HTML/CH.00.002.s5.htm

• http://phet.colorado.edu/simulations/sims.php?sim=Photoelectric_Effect• http://www.student.nada.kth.se/~f93-jhu/phys_sim/compton/Compton.htm• http://phet.colorado.edu/sims/rutherford-scattering/

rutherford-scattering.jnlp• http://phet.colorado.edu/sims/hydrogen-atom/hydrogen-atom.jnlp• http://www.nobelprize.org/nobel_prizes/physics/laureates/1929/broglie-

lecture.pdf• http://adsabs.harvard.edu/cgi-bin/nph-abs_connect?fforward=http://

dx.doi.org/10.1002/andp.19263851302• http://web.archive.org/web/20081217040121/http://home.tiscali.nl/

physis/HistoricPaper/Schroedinger/Schroedinger1926c.pdf• http://phet.colorado.edu/sims/quantum-tunneling/quantum-tunneling.jnlp• http://phet.colorado.edu/sims/fourier/fourier.jnlp