1. AA&A spring 20021

2. 2 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

3. AA&A spring 20023 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 motionsimulation

4. AA&A spring 20024 ****Questions on wave motion**** Go to the simulation website and:simulation website –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)

5. AA&A spring 20025 Wave motion  = frequency ( /s or Hertz [Hz]) s S = T =      (m) S = velocity of wave (m/s) T = period (s) = 1/

6. AA&A spring 20026 Electromagnetic waves S =              = 10 6 Hz = 300 m AM radio waves = 10 10 Hz = 3 cm x-band radar = 5.5 x 10 14 Hz = 550 nm green light = 3 x 10 18 Hz = 0.1 nm X-rays

7. AA&A spring 20027 An experiment Diffraction grating –Plastic sheet with grooves, 1 micron spacing –1,000 grooves/mm d sin  = Grating

8. AA&A spring 20028 Visible spectrum d sin  = connects to spectral colors definitive proof that light is wavelike goes black at ends because we can’t see, not because it’s not there!!

9. AA&A spring 20029 What is light? A stream of particles! 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 1.82.02.22.42.62.83.0 E in eV

10. AA&A spring 200210 Which, damn it, particle or wave? 1.82.02.22.42.62.83.0 E in eV Wave packet photon (E) wave ( ) E (in eV) = 1.24 x 10 3 / (in nm) (in nm)

11. AA&A spring 200211 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?

12. AA&A spring 200212 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

13. AA&A spring 200213 ****Questions on energy levels**** -15 -10 -7 -5 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)]

14. AA&A spring 200214 More complicated atoms

15. AA&A spring 200215 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 10 22 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