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5. The Nature of Light

5. The Nature of Light. Light travels in vacuum at 3.0 . 10 8 m/s Light is one form of electromagnetic radiation Continuous radiation: Based on temperature Wien ’ s Law & the Stefan-Boltzmann Law Light has both wave & particle properties Each element has unique spectral lines

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5. The Nature of Light

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  1. 5. The Nature of Light • Light travels in vacuum at 3.0 . 108 m/s • Light is one form of electromagnetic radiation • Continuous radiation: Based on temperature • Wien’s Law & the Stefan-Boltzmann Law • Light has both wave & particle properties • Each element has unique spectral lines • Atoms: A nucleus surrounded by electrons • Spectral lines: Electrons change energy levels • Spectral lines shift wavelength due to motion

  2. Does Light Travel Infinitely Fast? • Some ancient common experiences • Lightning & thunder • At minimum, light travels faster than easily measured • At maximum, light might travel infinitely fast • Galileo’s experiments • Human reflexes are much too slow • Human pulse ismuch too long • OlausRømer 1676 • Inconsistencies in occultations of Jupiter’s moons • Earlier than expectedwith Jupiter closer than average • Later than expected with Jupiter farther than average

  3. Occultation Occultation EMR Travels At Finite Speed

  4. Light Moves in Vacuum 3.0 . 108 m/s • Light travels at constant speed in vacuum • Recognized by Einstein as highest possible speed • Independent of the speed of any observer • That speed is…c…and is… “celeritas” c = 3.0 . 105 km/s c = 3.0 . 108 m/s c = 3.0 . 1010 cm/s • Light travels different speeds in different media • Air slows light a little Low density • Light bends/refracts a little as it enters the atmosphere • Glass slows light a lot High density • Light bends/refracts a lot as it enters a telescope lens

  5. “Light” is Electromagnetic Radiation • “Light” is one form of electromagnetic radiation • Electric & magnetic components are sine waves • Electric & magnetic components identical wavelengths • Electric & magnetic components perfectly synchronized • Various regions electromagnetic radiation • R Radio Longest λ’sLow energies • I Infrared • V Visible “Light” Medium energies • U Ultraviolet • X X-ray • G Gamma-ray Shortest λ’sHigh energies

  6. EMR: Electric & Magnetic Waves • Wave properties • Electric vector vibrates in a sine wave form vibrates in a single plane • Magnetic vector vibrates in a sine wave form vibrates perpendicular to e– vector vibrates synchronized w/e– vector

  7. Refraction of Sunlight By a Prism The “Celebrated Phenomenon of Colours” Red light is refracted least Blue light is refracted most

  8. Prisms Do Not Add Color to Light • Newton’s prism experiments • Isolate one color from sunlight using one prism • Passthat color through a second prism • No color is added

  9. The Electromagnetic Spectrum

  10. Emission & Absorption Spectra • Emission spectraBright = Hot Looking directly at a hot high-density object • Continuous  Hot high-density objects • Hot stars with no intervening interstellar gas clouds • Bright-line  Hot low-density objects • Hot interstellar gas clouds between any star & the Earth • Absorption spectra Dark = Cold Not looking directly at a hot high-density object • Dark-line Cool low-density objects • Cool interstellar gas clouds

  11. + = Continuous and Line Spectra Absorption from a cool low density object Emission from a hotEmission from a hot high density object low density object

  12. The Blackbody Concept • Blackbody: An ideal concept • Absorbs 100% of all wavelengths of incident EMR • All X-rays, visible light, radio waves… • Experience shows that this is impossible • Emits all absorbed energy as blackbody radiation • Radiation based exclusively on Kelvin temperature • Experience shows that this actually happens • Wien’s Law • Wavelength at which the most energy is produced • Stefan-Boltzmann Law • Total energy is proportional to T4

  13. “White” stars Our Sun “Red” stars Blackbody Curve: The Ideal

  14. Blackbody Curve: The Sun

  15. Wien’s Law • Blackbody radiation curves have one peak • This wavelength emits the most energy • This wavelength depends on Kelvin temperature lmax = Wavelength of maximum emission (meters) T = Temperature (kelvins) • maxis inversely proportional to Kelvin temp. • Higher temperature  Shorter wavelength

  16. The Stefan-Boltzmann Law • Blackbody radiation curves show energy flux • This energy flux depends on Kelvin temperature F = Energy flux (joules . m–2. sec–1 ) s = Constant = 5.67 . 10–8 W . m–2. K–4 TK = Temperature (kelvins) • Energy is directly proportional to TK4 • Raising TK by a factor of 10 raises energy by 10,000

  17. The Wave-Particle Nature of EMR • EMR behavior depends on the experiment • Wave experiment: EMR behaves like a wave • Young’s double-slit experiment • Particle experiment: EMR behaves like a particle • EMR as photons • A quantum amount of EMR energy • Energy = Planck’s Constant . Frequency • The photoelectric effect • Electron emission requires some minimum energy • Possible only if photons actually exist

  18. Each Element Has a Unique Spectrum • Every material has a unique spectral signature • Unique set of spectral lines • Whenhot, the spectral lines are bright • When cool, the spectral lines are dark • Each spectral line has a unique  Spectroscopy • Each spectral line emits a unique amount of energy • Kirchhoff’s Laws • Hot opaque objects: Continuous spectra • Classical blackbody radiation • Hot transparent objects: Bright-line spectra • Hot interstellar gas clouds with no continuous background • Cool transparent objects: Dark-line spectra • Cool interstellar gas clouds with a continuous background

  19. The Periodic Table of the Elements

  20. Spectra: The Hydrogen Family

  21. Spectra: The Helium Family

  22. Spectra: The Beryllium Family

  23. Spectra: The Boron Family

  24. Spectra: The Carbon Family

  25. Spectra: The Nitrogen Family

  26. Spectra: The Oxygen Family

  27. Spectra: The Fluorine Family

  28. The Bohr Model of the Atom • A central nucleus • One or more protons Atomic number • Determines the chemical properties (elements) • Zero or more neutrons Mass number • Determines the nuclear properties (isotopes) • Electron orbitals surround the nucleus • Neutral atoms: Number of p+ = Number of e– • Ionized atoms: Number of p+ ≠ Number of e– • Cations: One or more e– lost Net positive charge • Anions: One or more e– gained Net negative charge

  29. Bohr Model of the Hydrogen Atom Electron orbitals are not to scale

  30. Hydrogen Electron Transitions

  31. Electrons Jump Energy Levels • Electrons jumping energy levels produce lines • Hydrogen atom is the simplest of all • Lyman series: Ultraviolet spectrum • Balmer series: Visible spectrum • Paschen series: Infrared spectrum • All other atoms & elements are more complicated • More considerations about spectral lines • Each line has a different amount of energy • Energy = Planck’s constant . Frequency • Each line has a different probability of jumping • More jumps  More energy emitted  Brighter lines

  32. Spectra: Hydrogen Energy Levels

  33. The Doppler Effect • Effect Wavelength shift due to relative motion • Source & viewer moving closerBlue shift • Spectral lines shifted towardblue end of the spectrum • The spectral lines do not actually appear blue ! ! ! • Source & viewer moving fartherRed shift • Spectral lines shifted towardred end of the spectrum • The spectral lines do not actually appear red ! ! ! • Cause Relative motion of source & observer • Source & viewer moving closer • Waves compressed  Shorter wavelength  Blue shift • Source & viewer moving farther • Waves stretched  Longer wavelength  Red shift

  34. Doppler Shift: Stretching Waves Compressed wavelengths Stretched wavelengths Higher frequencies Lower frequencies Shift towardblue Shift towardred

  35. Light in vacuum at constant speed 3.0 . 108 m . sec–2 Light in other media moves slower Related generally to media density Light is one form of EMR Gamma rays X-rays Ultraviolet Visible Infrared Microwave / Radio Emission & absorption spectra Continuous Hot high density Bright line Hot low density Dark line Cool low density Blackbody concept Absorbs 100% of all wavelengths Emits 100% at specific wavelengths Wien’s Law Wavelength of maximum energy Stefan-Boltzmann Law Total energy produced Wave-particle duality of all EMR Behavior depends on experiment Photoelectric effect Unique sets of spectral lines Kirchhoff’s three laws Bohr’s mode of hydrogen Nucleus with orbitals Neutral & ionized atoms Electron energy jumps produce lines Doppler effect Relative convergence: Blue shift Relative divergence: Red shift Important Concepts

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