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

21 Sep 2000 ASTR103, GMU, Dr. Correll 1

Ch 04--Origin and Nature of LightCh 04--Origin and Nature of Light



• Blackbody Radiation• Discovering Spectra• Atoms and Spectra


Blackbody RadiationBlackbody 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


E&M RadiationE&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...


Atomic TheoryAtomic Theory

Atom - smallest unit displaying particular chemical and physical properties

Ernest Rutherford (1871-1937) - atom mostly empty spaceNucleus contains 99.98% of mass

Nucleus - central component of atomSize - about 10-4 of radius of electron orbitsMass - about 2000 times that of electronDensity - about 1012 to 1014 g/cm3

Electron clouds - clusters of electron orbits encircling nucleus


Atomic Theory Atomic Theory (cont.)(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 (1891-1974) as second primary particle in nucleus– No net electrical charge– Mass - approximately that of proton


Atomic Theory Atomic Theory (cont.)(cont.)


Electromagnetic ForceElectromagnetic 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?


How big is an atomHow big is an atom


Elements--different kinds of atomsElements--different kinds of atoms


States of MatterStates of Matter

• Solids - constituents, molecules or atoms, maintain reasonably permanent relation to each other– Typical separation is few constituent diameters– Solids rare in Universe


States of Matter States of Matter (cont.)(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



• Gases - constituents, molecules or atoms, maintain no relation relative to each other– Typical separation is many constituent diameters– Gases common in Universe


What about Ions?What about Ions?

++

++--oo oo

One electron systemOne electron system

NucleusNucleus

Ion: Helium Ion: Helium He He++ He II He IINucleus: Helium 4 Nucleus: Helium 4 44HeHe22

--

++

oo

electronelectron

protonproton

neutronneutron



• 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



• 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


Radiation LawsRadiation Laws

• Planck’s Law………………

• Stefan-Boltzmann Law…..

• Wien’s Displacement Law.



• Planck’s Law• Stephan-

Boltzmann Law• Wien’s

Displacement Law


Blackbody Radiation Blackbody Radiation (cont.)(cont.)

• Radiation emitted by stars tends to be much like that emitted by blackbody


Blackbody Radiation Blackbody Radiation (cont.)(cont.)





Discovering SpectraDiscovering Spectra

• Fraunhofer lines in the solar spectrum (1814)


Kirchoff-Bunsen ExperimentKirchoff-Bunsen Experiment

• Different chemicals have different spectra!


SpectrometrySpectrometry

• 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


SpectrumSpectrum


SpectraSpectra





Atoms and SpectraAtoms 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!


Bohr Model of the AtomBohr 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


Bohr Model of the AtomBohr 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


Hydrogen, for exampleHydrogen, for example


Doppler ShiftDoppler Shift• What happens to light when source and observer move relative to each other? Doppler

shifting of frequency!


Doppler ShiftDoppler Shift


Doppler ShiftDoppler 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!

Documents

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