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Exploring Photon Energy and Nuclear Reactions

Understand the complex nature of light and nuclear phenomena, from photon energy calculations to nuclear reactions and decay processes. Explore topics like photoelectric effect, emission spectra, nuclear decay, and wave-particle duality.

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Exploring Photon Energy and Nuclear Reactions

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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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