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Light, atomic structure, color

Light, atomic structure, color. Light is a kind of wave motion Electromagnetic spectrum/Visible spectrum Light is a kind of particle motion Resolution of contradiction Discrete spectral lines The Bohr hydrogen atom Spectral characterization of color. What is light? Some kind of wave!!.

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Light, atomic structure, color

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  1. Light, atomic structure, color • Light is a kind of wave motion • Electromagnetic spectrum/Visible spectrum • Light is a kind of particle motion • Resolution of contradiction • Discrete spectral lines • The Bohr hydrogen atom • Spectral characterization of color

  2. What is light? Some kind of wave!! • What is a wave? • Wave or pulse on a stretched spring • How is any point on the spring moving? • (Sea gull floating on water waves) • What IS moving along the spring? • Disturbance? Yes! • Energy? Yes! • “Stuff”? No! • Look at simulation of wave motion

  3. ****Questions on wave motion**** • Go to the simulation website and: • Launch a wave by dragging the right ball down • Change the frequency using the slider below the line • As the frequency is changed, • Does the speed of the wave change? • Does the wavelength of the wave change? • If you increase the frequency of a wave by a factor of ten, what happens to the wavelength? (stays same, increases by x 10, decreases by x 10, decreases by x 3) • FM radio is broadcast on a frequency of about 100 MHz (megahertz). What is the wavelength of these radio waves? (30 m, 3 m, 100 m, 30 cm) • (decreases by x 10; 3 m)

  4. Wave motion s  S =  T=  S = velocity of wave (m/s)  (m) = frequency ( /s or Hertz [Hz]) T = period (s) = 1/

  5. Electromagnetic waves = 106 Hz  = 300 m AM radio waves  = 1010 Hz  = 3 cm x-band radar  = 5.5 x 1014 Hz  = 550 nm green light  = 3 x 1018 Hz  = 0.1 nm X-rays S = 

  6. An experiment • Diffraction grating • Plastic sheet with grooves, 1 micron spacing • 1,000 grooves/mm • d sin q = l Grating

  7. Visible spectrum d sin q = l connects l to spectral colors definitive proof that light is wavelike goes black at ends because we can’t see, not because it’s not there!!

  8. What is light? A stream of particles! 3.0 2.8 2.6 2.4 2.2 2.0 1.8 • Photoelectric effect (see T&M) • Einstein’s Nobel prize • Unequivocal proof that light is particle-like • Massless particles of pure energy = photon • Units of energy: electron volt = eV • Energy of photon related to color E in eV

  9. Which, damn it, particle or wave? 3.0 2.8 2.6 2.4 2.2 2.0 1.8 E in eV Wave packet photon (E) <—> wave (l) E (in eV) = 1.24 x 103/l (in nm) l (in nm)

  10. Another experiment • Look at some gas discharge sources • “line spectrum” versus “continuous spectrum” • Every element has a unique line spectrum • Basis of chemical spectroscopic analysis • How might these line spectra connect with our picture of atoms?

  11. Bohr hydrogen atom Only a few allowed orbits (or energy states) for the electron Transitions among these with absorption or emission of a photon Each spectral line corresponds to one such transition

  12. ****Questions on energy levels**** 0 The sketch shows the energy levels of a one-electron atom in units of electron volts. If the electron is in the “ground state” (the blue circle) what energy photons is it able to absorb? If it happened to be in the -7 eV level, (the yellow circle) what energy photons could it absorb?, emit? [5,8,10, or 15 eV photons]; [(2 or 7 eV photons), (3 or 8 eV photons)] -5 -7 -10 -15

  13. More complicated atoms

  14. What have we seen? • Light: wave or particle? Wave packets! • Wavelength <—> Energy relation • Electromagnetic spectrum: • 60 Hz household current to 10 GeV gamma rays is range of x 1022 in energy/wavelength • Visible spectrum • Range of x 2 • Bohr atom • Discrete energy states • Photon emission/absorption <—> transitions between states • Spectral (physicist’s) characterization of color

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