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Ch 04--Origin and Nature of Light 21 Sep 2000 ASTR103, GMU, Dr. Correll 1 Ch 04--Origin and Nature of Light • Blackbody Radiation • Discovering Spectra • Atoms and Spectra 21 Sep 2000 ASTR103, GMU, Dr. Correll 2 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 2000 ASTR103, GMU, Dr. Correll 3 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 2000 ASTR103, GMU, Dr. Correll 4 Atomic Theory Atom - smallest unit displaying particular chemical and physical properties Ernest Rutherford (1871-1937) - atom mostly empty space Nucleus contains 99.98% of mass Nucleus - central component of atom Size - about 10-4 of radius of electron orbits Mass - about 2000 times that of electron Density - about 1012 to 1014 g/cm3 Electron clouds - clusters of electron orbits encircling nucleus 21 Sep 2000 ASTR103, GMU, Dr. Correll 5 Atomic Theory (cont.) • Electron identified 1897, J. J. Thomson (1856-1940) – 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 - 2000 times that of electron • Neutron identified 1932 by James Chadwick (18911974) as second primary particle in nucleus – No net electrical charge – Mass - approximately that of proton 21 Sep 2000 ASTR103, GMU, Dr. Correll 6 Atomic Theory (cont.) 21 Sep 2000 ASTR103, GMU, Dr. Correll 7 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 2000 ASTR103, GMU, Dr. Correll 8 How big is an atom 21 Sep 2000 ASTR103, GMU, Dr. Correll 9 Elements--different kinds of atoms 21 Sep 2000 ASTR103, GMU, Dr. Correll 10 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 2000 ASTR103, GMU, Dr. Correll 11 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 2000 ASTR103, GMU, Dr. Correll 12 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 2000 ASTR103, GMU, Dr. Correll 13 What about Ions? - Ion: Helium He+ He II Nucleus: Helium 4 4He2 - + o electron proton neutron + o + o Nucleus One electron system 21 Sep 2000 ASTR103, GMU, Dr. Correll 14 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 2000 ASTR103, GMU, Dr. Correll 15 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 2000 ASTR103, GMU, Dr. Correll 16 Radiation Laws • Planck’s Law……………… • Stefan-Boltzmann Law….. • Wien’s Displacement Law. 21 Sep 2000 ASTR103, GMU, Dr. Correll 17 Blackbody Radiation • Planck’s Law • StephanBoltzmann Law • Wien’s Displacement Law 21 Sep 2000 ASTR103, GMU, Dr. Correll 18 Blackbody Radiation (cont.) • Radiation emitted by stars tends to be much like that emitted by blackbody 21 Sep 2000 ASTR103, GMU, Dr. Correll 19 Blackbody Radiation (cont.) 21 Sep 2000 ASTR103, GMU, Dr. Correll 20 Ch 04--Origin and Nature of Light • Blackbody Radiation • Discovering Spectra • Atoms and Spectra 21 Sep 2000 ASTR103, GMU, Dr. Correll 21 Discovering Spectra • Fraunhofer lines in the solar spectrum (1814) 21 Sep 2000 ASTR103, GMU, Dr. Correll 22 Kirchoff-Bunsen Experiment • Different chemicals have different spectra! 21 Sep 2000 ASTR103, GMU, Dr. Correll 23 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 2000 ASTR103, GMU, Dr. Correll 24 Spectrum 21 Sep 2000 ASTR103, GMU, Dr. Correll 25 Spectra 21 Sep 2000 ASTR103, GMU, Dr. Correll 26 Ch 04--Origin and Nature of Light • Blackbody Radiation • Discovering Spectra • Atoms and Spectra 21 Sep 2000 ASTR103, GMU, Dr. Correll 27 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 2000 ASTR103, GMU, Dr. Correll 28 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 2000 ASTR103, GMU, Dr. Correll 29 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 2000 ASTR103, GMU, Dr. Correll 30 Hydrogen, for example 21 Sep 2000 ASTR103, GMU, Dr. Correll 31 Doppler Shift • What happens to light when source and observer move relative to each other? Doppler shifting of frequency! 21 Sep 2000 ASTR103, GMU, Dr. Correll 32 Doppler Shift 21 Sep 2000 ASTR103, GMU, Dr. Correll 33 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! 21 Sep 2000 ASTR103, GMU, Dr. Correll 34