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MASON ASTR 113 - Light and Other Electromagnetic Radiation

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11Light and Other Electromagnetic Radiation2Topics Covered in Chapter1.Structure of Atoms 2.Origins of Electromagnetic Radiation3.Objects with Different Temperature and their Electromagnetic Radiation4.Kirchoff’s Spectral Laws5.Bohr’s Model of the Atom6.Doppler Effect3A Subatomic Interlude4A Subatomic Interlude II5A Subatomic Interlude III6A Subatomic Interlude IIII27A Subatomic Interlude V• Neutrinos are produced in the “Weak Interaction”, for example– Neutrinos from the earth• natural radioactivity– “Man-made” neutrinos• accelerators, nuclear power plants.– Astrophysical neutrinos• Solar neutrinos• Atmospheric neutrinos• Relic neutrinos– left over from the big bang.8Neutrino DetectionDetecting neutrinos requires a differentkind of a detector.9Neutrino Factoids• The earth receives about 40 billion neutrinos per second per cm2from the sun.– About 100 times that amount are passing through us from the big bang.• This works out to about 330 neutrinos in every cm3of the universe!• By comparison there are about 0.0000005 protons per cm3in the universe.• Your own body emits about 340 million neutrinos per day from 40K.• Neutrinos don’t do much when passing through matter.– Thus, it is very difficult to observe neutrinos.10Neutrino Detection II• Neutrinos are observed by detecting the product of their interaction with matter.eElectronMuon11Neutrinos reveal information about the Sun’score—and have surprises of their own• Neutrinos emitted in thermonuclear reactions in the Sun’s core were detected, but in smaller numbers (1/3) than expected• Recent neutrino experiments explain why this is so– Based upon conversion of electron neutrino to tau neutrino12Determining the Speed of Light• Galileo tried unsuccessfully to determine the speed of light using an assistant with a lantern on a distant hilltop313Light travels through empty space at a speedof 300,000 km/s• In 1676, Danish astronomer Olaus Rømer discovered that the exact time of eclipses of Jupiter’s moons depended on the distance of Jupiter to Earth • This happens because it takes varying times for light to travel the varying distance between Earth and Jupiter• Using d=rt with a known distance and a measured time gave the speed (rate) of the light14• In 1850 Fizeau and Foucalt also experimented with light by bouncing it off a rotating mirror and measuring time• The light returned to its source at a slightly different position because the mirror has moved during the time light was traveling• d=rt again gave c15Light is electromagnetic radiationand is characterized by its wavelength ()16Wavelength and Frequency17The Nature of Light• In the 1860s, the Scottish mathematician and physicist James Clerk Maxwell succeeded in describing all the basic properties of electricity and magnetism in four equations• This mathematical achievement demonstrated that electric and magnetic forces are really two aspects of the same phenomenon, which we now call electromagnetism18Electromagnetism• Electricity according to Gauss– relates electricity to electric charge• Faraday’s Law– relates electric fields to magnetic fields• Magnetism according to Gauss– relates magnetism to electricity419Maxwell’s Equations• Ampere-Maxwell Law– relates magnetic field to electricity•Maxwell– unifies electricity and magnetism into electromagnetism20• Because of its electric and magnetic properties, light is also called electromagnetic radiation• Visible light falls in the 400 to 700 nm range• Stars, galaxies and other objects emit light in all wavelengths21Three Temperature Scales22An opaque object emits electromagnetic radiationaccording to its temperature23A person in infrared-color coded image-red is hottest24Wien’s law and the Stefan-Boltzmann law are useful tools for analyzing glowing objects like stars• A blackbody is a hypothetical object that is a perfect absorber of electromagnetic radiation at all wavelengths• Stars closely approximate the behavior of blackbodies, as do other hot, dense objects• The intensities of radiation emitted at various wavelengths by a blackbody at a given temperature are shown by a blackbody curve525Wien’s LawWien’s law states that the dominant wavelength at which a blackbody emits electromagnetic radiation is inversely proportional to the Kelvin temperature of the object2627Stefan-Boltzmann Law• The Stefan-Boltzmann law states that a blackbody radiates electromagnetic waves with a total energy flux E directly proportional to the fourth power of the Kelvin temperature T of the object:E = T42829Light has properties of both waves and particles• Newton thought light was in the form of little packets of energycalled photons and subsequent experiments with blackbody radiation indicate it has particle-like properties• Young’s Double-Slit Experiment indicated light behaved as a wave• Light has a dual personality; it behaves as a stream of particlelike photons, but each photon has wavelike properties30Light, Photons and Planck• Planck’s law relates the energy of a photon to its frequency or wavelengthE = energy of a photonh = Planck’s constantc = speed of light = wavelength of light• The value of the constant h in this equation, called Planck’s constant, has been shown in laboratory experiments to beh = 6.625 x 10–34J s631Prelude to the Bohr Model of the Atom• The Photoelectric Effect– experiment explained by Einstein, but performed by others• What caused this strange result?• This is what Einstein won the Nobel Prize for32Chemists’ Observations33 34Each chemical element produces its own unique set of spectral lines35Kirchhoff’s Laws36Kirchoff’s First Spectral Law• Any hot body produces a continuous spectrum– if it’s hot enough it looks something like this– digitally like thisWavelengthIntensity737Kirchoff’s Second Spectral Law• Any gas to which energy is applied, either as heat or a high voltage, will produce an emission line spectrum like this– or digitally like thisWavelengthIntensity38Kirchoff’s Third Spectral Law• Any gas placed between a continuous spectrum source and the observer will produce a absorption line spectrum like this– or digitally like thisWavelengthIntensity39Astronomers’ Observations4041 42An atom consists of a small, dense nucleussurrounded by electrons• An atom has a small dense nucleus composed of protons and neutrons•


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