Physics 202: Introduction to Astronomy – Lecture 5 Carsten Denker Physics Department Center for Solar–Terrestrial Research

February 1, 2006Center for Solar-Terrestrial Research Chapter 2.1 – 2.6  Light and radiation  Spectrum: radio, infrared, visible, ultra- violet, X-rays, gamma-rays  Waves: wavelength, frequency, amplitude, and period  Electromagnetic waves  Electromagnetism  Blackbody radiation  Radiation laws  Spectroscopy  Emission and absorption lines  Spectral line formation  Bohr model of atoms  Hydrogen spectrum  Kirchoff’s laws

February 1, 2006Center for Solar-Terrestrial Research Coulomb’s Law Superposition principle Coulomb’s law Gravitational force Coulomb’s law

February 1, 2006Center for Solar-Terrestrial Research Electromagnetic Waves

February 1, 2006Center for Solar-Terrestrial Research The Interaction of Light and Matter

February 1, 2006Center for Solar-Terrestrial Research Infrared Radiation  In 1800, William Herschel (1738 –1822) extended Newton's experiment of separating chromatic light components via refraction through a glass prism by demonstrating that invisible "rays" existed beyond the red end of the solar spectrum.

February 1, 2006Center for Solar-Terrestrial Research Electromagnetic Spectrum

February 1, 2006Center for Solar-Terrestrial Research Kirchhoff’s Laws  A hot (< 0 K), dense gas or solid object produces 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.

February 1, 2006Center for Solar-Terrestrial Research Spectroscopy  The English chemist and physicist William Hyde Wollaston (1766 – 1828) noticed dark lines in the spectrum of the Sun while investigating the refractive properties of various transparent substances  Joseph von Fraunhofer ( ) independently rediscovered the “dark lines” in the solar spectrum

February 1, 2006Center for Solar-Terrestrial Research Spectroscopy  Prisms  Diffraction gratings Transmission grating Reflection grating Resolving power

February 1, 2006Center for Solar-Terrestrial Research The Bohr Model of the Atom  Wave–particle duality of light  Rutherford 1911   Au: It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15–inch shell at a piece of tissue paper and it came back an hit you.  discovery of a minute, massive, positively charged atomic nucleus  Proton: m p = 1836  m e

February 1, 2006Center for Solar-Terrestrial Research Hydrogen Atom m = 1UV [122, 103, 97, …] nmLyman m = 2Visible [656, 486, 434, …] nmBalmer m = 3IR [1875, 1282, 1094, …] nmPaschen m = 4IR [4051, 2625, 2165, …] nmBrackett m = 5IR [7458, 4652, …] nmPfundt Planetary model of the hydrogen atom?

February 1, 2006Center for Solar-Terrestrial Research Kirchhoff’s Laws Revisited  A hot, dense gas or hot solid object produces a continuous spectrum with no dark spectral lines. This is the continuous spectrum of black body radiation, described by the Planck functions B (T) and B (T), emitted at any temperature above absolute zero. The wavelength max at which the Planck function B (T) obtains its maximum is given by Wien’s displacement law.

February 1, 2006Center for Solar-Terrestrial Research Kirchhoff’s Laws Revisited (cont.)  A hot, diffuse gas produces bright emission lines. Emission lines are produced when an electron makes a downward transition from a higher to a lower orbit. The energy lost by the electron is carried away by the photon.  A cool, diffuse gas in front of a source of continuous spectrum produces dark absorption lines in the continuous spectrum. Absorption lines are produced when an electron makes a transition from a lower to a higher orbit. If the incident photon in the continuous spectrum has exactly the right amount of energy, equal to the difference in energy between a higher orbit and the electron’s initial orbit, the photon is absorbed by the atom and the electron makes an upward transition to the higher orbit.