2. 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

3. But light is also a wave! Travels at constant speed c in a vacuum. c = f –c: 3 x 10 8 m/s –  wavelength (m) – f: frequency (Hz)

4. 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)

5. 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!

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

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

8. 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.

9. Voltage current for different intensities of light. V i VsVs I1I1 I2I2 I3I3 I 3 > I 2 > I 1 Stopping potential is unaffected!

10. Voltage versus current for different frequencies of light. V i V s,1 f1f1 f2f2 V s,2 f3f3 V s,3 f 3 > f 2 > f 1 Stopping potential becomes more negative at higher frequencies!

11. Photoelectric Effect E photon = K max + W o –E photon = hf (Planck’s equation) –K max : maximum kinetic energy of electrons –W o : binding energy or “work function” hf = K max + W o

12. Graph of Photoelectric Equation f K max hf = K max + W o K max = hf - W o y = mx + b slope = h (Planck’s Constant) W o  (binding energy) Cut-off frequency

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

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

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

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

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

18. Isotope characteristics differ U 238 92 U 235 92

19. Binding energy Energy released when a nucleus is formed from protons and neutrons. Mass is lost. E = mc 2 –where m is the lost mass

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

21. Nuclear reactions Nuclear Decay –Alpha decay –Beta decay Beta Minus Positron Fission Fusion

22. Decay Particles Alpha Beta Positron He 4 2 e 0 e 0 1

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

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

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

26. 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.

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

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

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

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

31. Energy Particle –E = K + U Photon –E = hf

32. Momentum Particle –p = mv Photon –p = h/

33. Wavelength Photon –  = c/f Particle – = h/p – deBroglie wavelength

34. 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.

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