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photons

photons. Physics 100 Chapt 21. Photoelectric effect. cathode. anode. Vacuum tube. Photoelectric effect. Vacuum tube. Experimental results. Electron KE ( electron Volts). For light freq below f 0 , no electrons leave the cathode. f 0. Even if the light Is very intense.

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photons

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  1. photons Physics 100 Chapt 21

  2. Photoelectric effect cathode anode Vacuum tube

  3. Photoelectric effect Vacuum tube

  4. Experimental results Electron KE (electron Volts) For light freq below f0, no electrons leave the cathode f0 Even if the light Is very intense 0 0.5 1.0 1.5

  5. Experimental results For light freq above f0, the KE of electrons that leave the cathode increases with increasing freq Electron KE (electron Volts) f0 But does not change With light intensity 0 0.5 1.0 1.5

  6. What does Maxwell’s theory say? Electrons in cathode are accelerated by the E-field of the light wave E E E

  7. More intense light hasbigger E-fields E E E And, therefore Larger acceleration

  8. Electron KE should depend on E-field strength light intensity Electron’s motion Not what is observed

  9. But that’s not what is observed Above f0,the KE only depends on freq, & not on the light’s intensity Electron KE (electron Volts) Below f0, no electrons jump out of the cathode no matter what the light’s intensity is f0 0 0.5 1.0 1.5

  10. Einstein’s explanation Light is comprised of particle-like quanta each with energyEquant = hf The quanta collide with electrons & Transfer all their energy to them Each electron needs a minimum energy to escape the cathode. This is called f If Equant is less than f, the electron can’t escape If Equant is greater than f, the electron escapes & the quantum energy in excess of f becomes electron KE f KEelectron = hf - f

  11. Light quanta  “photons” Einstein’s light quanta were given the name “photons” by Arthur Compton

  12. Photon Energyfor red light Red light: f = 4.0x1014Hz Ephoton = hf = (6.6x10-34 Js)x(4.0x1014Hz) = 26x10-20 J = (6.6x4.0)x10-34+14 J 1eV 1.6x10-19 J x = 2.6x10-19 J 2.6 1.6 eV = =1.6 eV

  13. Photon Energiesfor visible light color: freq Equant = hf Red 4.0x1014Hz 2.6x10-19J 1.6eV Yellow 5.0x1014Hz 3.3x10-19J 2.1eV Green 6.0x1014Hz 4.0x10-19J 2.5eV Blue 6.7x1014Hz 4.4x10-19J 2.8eV Violet 7.5x1014Hz 5.0x10-19J 3.1eV

  14. Producing photoelectrons with photons Clears the barrier with energy to spare Not enough energy to get over the barrier - - Red photon 1.6eV KE=0.7eV 2.8eV outside of the metal - Surface barrier - - f=2.1eV - Blue photon inside the metal

  15. For E Electron KE (electron Volts) violet blue yellow red KE KE 0 0.5 1.0 1.5

  16. Photons are weird particles v=c (always) 1 1 – v2/c2 1 1 – c2/c2 = 1 1 – 1 =  (always) =

  17. What is the photon’s rest mass? E c2 E=mc2  m= m  m g m =gm0 = =0  m0=  Rest mass=0 m0=0

  18. Photon’s momentum For any particle: p=mv E c2 & v=c for a photon: m= E c E c2 p = c =

  19. Photon energy & momentum E = hf h l E c hf c = p= = c f 1 l f c Wavelength:l =  =

  20. “particles” of light h l p = E=hf

  21. Two body collisions conservation of momentum

  22. Compton scattering Scatter X-rays from electrons p=h/li - Recoil electron & scattered photon conserve momentum p=h/lf

  23. Compton’s expt proved the existence of photons & won him the 1927 Nobel Prize (Physics)

  24. Photon “spectrum” Infra- red Ultra- violet radio waves micro waves X-rays g-rays TV/FM AM 4x10-11eV 4x10-7eV 4x10-3eV 4eV 4x103eV 4x106eV visible light 1.6 – 3.1eV

  25. Wave? Particles?? Physics 100 Chapt 22

  26. Maxwell E B Light is a wave of oscillating E- and B-fields James Clerk Maxwell

  27. Einstein Light is comprised of particle-like quanta called photons h l p = E=hf

  28. Who’s right?? Waves explain diffraction & interference Photons explain photoelectric effect & Compton scattering

  29. Impossible to explain interference with particles With 2 slits open no light goes here Block off one slit Now light can go here

  30. Impossible to explain PE-effectand Compton scattering with waves Electron KE (electron Volts) violet blue yellow red 0.5 1.0 1.5

  31. Make an interferencepattern with low intensity light One photon at a time goes through the two-slit apparatus

  32. "Wave-Particle“ duality -Light behaves like a wave when it propagates through space -And as a particle when it interacts with matter

  33. Photon photography

  34. Photoelectric effect Vacuum tube

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