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The photon

The photon. A “particle” of light A “quantum” of light energy The energy of a given photon depends on the frequency (color) of the light. But light is also a wave!. Travels at constant speed c in a vacuum. c = l f c: 3 x 10 8 m/s l: wavelength (m) f: frequency (Hz).

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The photon

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  1. The photon • A “particle” of light • A “quantum” of light energy • The energy of a given photon depends on the frequency (color) of the light

  2. But light is also a wave! • Travels at constant speed c in a vacuum. • c = lf • c: 3 x 108m/s • l: wavelength (m) • f: frequency (Hz)

  3. Calculating photon energy • E = hf • E: energy (J or eV) • h: Planck’s constant • 6.62510-34 J s or 4.14 10-15 eV s • f: frequency of light (s-1, Hz)

  4. The “electron-volt” (eV)is an energy unit • Useful on the atomic level. • If a moving electron is stopped by 1 V of electric potential, we say it has 1 electron-volt (or 1 eV) of kinetic energy!

  5. Converting eV to Joules (J) 1 eV = 1.60210-19J

  6. light light Photoelectric Effect experiment Collector (-) Photo- Diode (+) At a certain voltage, Vs, the current can’t flow anymore! e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- V e- e- A e- e- e- e- e-

  7. Anomalous Behavior in Photoelectric Effect • Voltage necessary to stop electrons is independent of intensity (brightness) of light. • Photoelectrons are not released below a certain frequency, regardless of intensity of light. • The release of photoelectrons is instantaneous, even in very feeble light, provided the frequency is above the cutoff.

  8. I3 I2 I1 Vs Voltage current for different intensities of light. i I3 > I2 > I1 V Stopping potential is unaffected!

  9. f3 f2 f1 Vs,1 Vs,3 Vs,2 Voltage versus current for different frequencies of light. f3 > f2 > f1 i V Stopping potential becomes more negative at higher frequencies!

  10. Photoelectric Effect • Ephoton = Kmax + Wo • Ephoton = hf (Planck’s equation) • Kmax: maximum kinetic energy of electrons • Wo: binding energy or “work function” • hf = Kmax + Wo

  11. hf = Kmax+ Wo Kmax = hf - Wo y = mx + b slope = h (Planck’s Constant) Cut-off frequency Wo(binding energy) Graph of Photoelectric Equation Kmax f

  12. 0 eV DE hf Ground state -10 eV Absorption Spectrum Photon is absorbed and excites atom to higher quantum energy state.

  13. ionized 0 eV -10 eV Absorption Spectrum Absorption spectra always involve atoms going up in energy level.

  14. 0 eV DE hf Excited state -10 eV Emission Spectrum Photon is emitted and atom drops to lower quantum energy state.

  15. ionized 0 eV -10 eV Emission Spectrum Emission spectra always involve atoms going down in energy level.

  16. Atomic mass: protons plus neutrons 12 C Element name 6 Atomic number: protons A typical nucleus

  17. 238 235 U U 92 92 Isotope characteristics differ Fission! Low Radioactivity

  18. Binding energy • Energy released when a nucleus is formed from protons and neutrons. • Mass is lost. • E = mc2 • where m is the lost mass

  19. 1 p 1 1 n 0 Nuclear Particles • Nucleons • Proton • Charge: +e • Mass: 1 amu • Neutron • Charge: 0 • Mass: 1 amu

  20. Nuclear reactions • Nuclear Decay • Alpha decay • Beta decay • Beta Minus • Positron • Fission • Fusion

  21. 4 He 2 0 0 e e -1 1 Decay Particles • Alpha • Beta • Positron

  22. 4 He 2 226 222 Ra Rn 88 86 Alpha Decay • Occurs only with very heavy elements. • Nucleus too large to be stable.

  23. 0 e n -1 40 40 K Ca 19 20 anti- neutrino Beta Decay • Occurs with elements that have too many neutrons for the nucleus to be stable.

  24. 0 e n 1 2 2 He H 2 1 neutrino Positron Decay • Occurs with elements that have too many protons for the nucleus to be stable.

  25. Neutrino and Anti-Neutrino • Proposed to make beta and positron decay obey conservation of energy. • No mass, no charge. • Energy and spin. • Does not react easily with matter. • Hard to detect.

  26. Gamma Radiation, g • Released by atoms which have undergone a nuclear reaction. • Results when excited nuclei return to ground state. • High energy! E = hf!

  27. 1 1 n n 0 0 144 239 92 Pu Sr Ba 4 94 38 56 Fission • Occurs only with very heavy elements. • Nucleus too large to be stable. • Induced by neutrons.

  28. 1 1 H H 2 He 1 1 2 Fusion • The largest amount of energy available. • Energy produced in the sun. • Fusion of light elements results in non-radioactive waste.

  29. Summary of Wave-Particle Duality Waves are particles and particles are waves

  30. Energy • Particle • E = K + U • Photon • E = hf

  31. Momentum • Particle • p = mv • Photon • p = h/l

  32. Wavelength • Photon • l = c/f • Particle • l = h/p • deBroglie wavelength

  33. Compton Scattering • Proof of the momentum of photons. • High-energy photons collided with electrons. • Conservation of momentum. • Scattered photons examined to determine loss of momentum.

  34. Davisson-Germer Experiement • Verified that electrons have wave properties by proving that they diffract. • Electron diffraction

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