21 Sep 2000ASTR103, GMU, Dr. Correll1 Ch 04--Origin and Nature of Light

21 Sep 2000ASTR103, GMU, Dr. Correll2 Ch 04--Origin and Nature of Light Blackbody Radiation Discovering Spectra Atoms and Spectra

21 Sep 2000ASTR103, GMU, Dr. Correll3 Blackbody Radiation “Glowing red hot”--blackbody radiation is the name given to electromagnetic radiation emitted by an heated object. –Solids and dense gases give off blackbody radiation

21 Sep 2000ASTR103, GMU, Dr. Correll4 E&M Radiation All matter is constantly in motion at the atomic level The higher the temperature, the more motion The more motion, the more light that is radiated Let’s digress to consider atomic theory...

21 Sep 2000ASTR103, GMU, Dr. Correll5 Atomic Theory Atom - smallest unit displaying particular chemical and physical properties Ernest Rutherford ( ) - atom mostly empty space Nucleus contains 99.98% of mass Nucleus - central component of atom Size - about of radius of electron orbits Mass - about 2000 times that of electron Density - about to g/cm 3 Electron clouds - clusters of electron orbits encircling nucleus

21 Sep 2000ASTR103, GMU, Dr. Correll6 Atomic Theory (cont.) Electron identified 1897, J. J. Thomson ( ) –Unit of negative electrical charge –Mass - about 1/2000 that of proton Light mass makes them fast! Proton identified 1919 by Rutherford as principal constituent of nucleus –Unit of positive electrical charge –Mass times that of electron Neutron identified 1932 by James Chadwick ( ) as second primary particle in nucleus –No net electrical charge –Mass - approximately that of proton

21 Sep 2000ASTR103, GMU, Dr. Correll7 Atomic Theory (cont.)

21 Sep 2000ASTR103, GMU, Dr. Correll8 Electromagnetic Force Lorentz Force –q is charge for particle one and two –r is seperation –k is constant of proportionality –notice negative sign! Does this equation look familiar?

21 Sep 2000ASTR103, GMU, Dr. Correll9 How big is an atom

21 Sep 2000ASTR103, GMU, Dr. Correll10 Elements--different kinds of atoms

21 Sep 2000ASTR103, GMU, Dr. Correll11 States of Matter Solids - constituents, molecules or atoms, maintain reasonably permanent relation to each other –Typical separation is few constituent diameters –Solids rare in Universe

21 Sep 2000ASTR103, GMU, Dr. Correll12 States of Matter (cont.) Liquids - constituents, molecules or atoms, maintain only temporary relation to each other –Typical separation is several constituent diameters –Liquids are non-existent for all practical purposes

21 Sep 2000ASTR103, GMU, Dr. Correll13 States of Matter (cont.) Gases - constituents, molecules or atoms, maintain no relation relative to each other –Typical separation is many constituent diameters –Gases common in Universe

21 Sep 2000ASTR103, GMU, Dr. Correll14 What about Ions? oo One electron system Nucleus Ion: Helium  He +  He II Nucleus: Helium 4  4 He o electron proton neutron

21 Sep 2000ASTR103, GMU, Dr. Correll15 States of Matter (cont.) Plasmas - state similar to gases, but atoms are ionized –One or more electrons stripped off atom –Most visible matter in Universe in form of a plasma –Highly ionized plasmas predominate

21 Sep 2000ASTR103, GMU, Dr. Correll16 Blackbody Radiation Planck’s Law –1900, Max Planck derived mathematical law describing distribution of brightness in blackbody spectrum Stefan-Boltzmann Law –Energy emission is greater at every wavelength as temperature increases; total amount of radiant energy emitted increases with increasing temperature Wien’s Displacement Law –Maximum emission found toward shorter wavelengths (blue end of spectrum) as temperature increases

21 Sep 2000ASTR103, GMU, Dr. Correll17 Radiation Laws Planck’s Law……………… Stefan-Boltzmann Law….. Wien’s Displacement Law.

21 Sep 2000ASTR103, GMU, Dr. Correll18 Blackbody Radiation Planck’s Law Stephan- Boltzmann Law Wien’s Displacement Law

21 Sep 2000ASTR103, GMU, Dr. Correll19 Blackbody Radiation (cont.) Radiation emitted by stars tends to be much like that emitted by blackbody

21 Sep 2000ASTR103, GMU, Dr. Correll20 Blackbody Radiation (cont.)

21 Sep 2000ASTR103, GMU, Dr. Correll21 Ch 04--Origin and Nature of Light Blackbody Radiation Discovering Spectra Atoms and Spectra

21 Sep 2000ASTR103, GMU, Dr. Correll22 Discovering Spectra Fraunhofer lines in the solar spectrum (1814)

21 Sep 2000ASTR103, GMU, Dr. Correll23 Kirchoff-Bunsen Experiment Different chemicals have different spectra!

21 Sep 2000ASTR103, GMU, Dr. Correll24 Spectrometry Spectrometry--a very important tool in astronomy! –Spectrum recorded at the focal plane of a telescope –spectra give information about the composition, temperature and pressure of the astronomical object

21 Sep 2000ASTR103, GMU, Dr. Correll25 Spectrum

21 Sep 2000ASTR103, GMU, Dr. Correll26 Spectra

21 Sep 2000ASTR103, GMU, Dr. Correll27 Ch 04--Origin and Nature of Light Blackbody Radiation Discovering Spectra Atoms and Spectra

21 Sep 2000ASTR103, GMU, Dr. Correll28 Atoms and Spectra Until now, we’ve talked about atoms as little billiard balls--nuclei dragging electrons around to produce E&M radiation But the structure we see in the spectra of light indicates that the structure of these atoms has some interesting features –This leads to the quantum theory of the atom!

21 Sep 2000ASTR103, GMU, Dr. Correll29 Bohr Model of the Atom Bohr Model--Bohr hypothesized that electrons orbit at discrete levels, jumping up or down in energy levels (1911) –Planck and Einstein had earlier proposed quantum ideas about light

21 Sep 2000ASTR103, GMU, Dr. Correll30 Bohr Model of the Atom Electrons change energy levels in an atom by absorbing or emitting a photon! Electrons tend to settle to the lowest energy level, the ground state

21 Sep 2000ASTR103, GMU, Dr. Correll31 Hydrogen, for example

21 Sep 2000ASTR103, GMU, Dr. Correll32 Doppler Shift What happens to light when source and observer move relative to each other? Doppler shifting of frequency!

21 Sep 2000ASTR103, GMU, Dr. Correll33 Doppler Shift

21 Sep 2000ASTR103, GMU, Dr. Correll34 Doppler Shift Motion of source away form observer causes a red shift Motion of source towards observer causes a blue shift Motion lateral to observer gives no shift!