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PH 103

PH 103. Dr. Cecilia Vogel Lecture 17. Review. Relativistic velocity transformation. Quantum Mechanics What is quantization? Photon Two pieces of evidence: blackbody radiation photoelectric effect. Outline. Familiar Quantization. Electric charge

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PH 103

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  1. PH 103 Dr. Cecilia Vogel Lecture 17

  2. Review • Relativistic velocity transformation • Quantum Mechanics • What is quantization? • Photon • Two pieces of evidence: • blackbody radiation • photoelectric effect Outline

  3. Familiar Quantization • Electric charge • At first glance, it seems you could have any charge (any value in Coulombs) • If very accurately measured, find charge is always integer*1.602X10-19C = Ne • Charge is due to individual, indivisible electrons • Charge is quantized • in other words, only certain values are possible. • Other familiar examples: • individual molecules in “continuous” material • individual pixels in “continuous” image

  4. Photon • What is a photon? • Individual, indivisible bit of light energy • a massless particle • How much energy? • For light of frequency, f, • each photon has an energy, Ephoton = hf • example: • UV light with frequency 1X1015Hz • One photon has energy (6.626X10-34 Js)(1X1015Hz) • One photon has energy 6.626X10-19 J

  5. Photon • Each photon has an energy, Ephoton = hf • N photons have energy E= Nhf • N = integer!! • Cannot have half a photon! • example: • UV light with frequency 1X1015Hz • One photon has energy 6.626X10-19 J • Two photons have energy 1.3252X10-18 J • How many photons have 100 J of energy? • N=E/hf = 1.5X1020 photons • So 1.5X1020 photons/s = 100 W

  6. Evidence for photons Evidence that light is made up of massless particles called photons: • In blackbody radiation, light is emitted one photon at a time. • In the photoelectric effect, light is absorbed one photon at a time. • In Compton scattering, light collides with an electron one photon at a time. • … and so much more

  7. Blackbody (Thermal) Radiation • A blackbody is an idealization • ignore characteristic spectrum • for simplification • Blackbody thermal radiation • light and other EM waves • given off by an object due to temperature • glowing

  8. Familiar Thermal Radiation • no visible • infrared/heat • Warm • Red hot Power spectrum • lots infrared • some red • White hot • all colors • IR & UV

  9. Blackbody Radiation • Peak of emission is at wavelength: Power spectrum • Plug in T in Kelvin

  10. Failure of Classical Theory • Physicists attempted to describe radiation • using wave theory of light • andclassical thermal physics • Predicted • lots of high frequency light • at all temps! • called the “ultraviolet catastrophe”

  11. Photon Theory • Max Plank’s theory: • Light must be given off as individual photons (he didn’t use that term) • Each photon has energy E= hf. • High frequency light made up of high energy photons. • Requires a lot of energy to produce even one. • High-frequency photons are harder to produce, so less likely to be produced. • In fact as f, E • not just unlikely, impossible to produce one • power spectrum goes to zero as f

  12. Warning • Never believe a theory just because it fits the experiment it was made up to fit! • It should explain other experiments. • Or make predictions that can be verified.

  13. -ELECTRIC PHOTO- Photoelectric Effect • Light strikes a metal, the light is absorbed, knocking electrons off the metal’s surface. • Light energy converted to electrical energy. • The electrons could absorb energy from a wave or a particle of light • but wave theory can’t explain the details... • Particle (photon) theory can.

  14. PEE – Failure of Classical Theory • If light’s frequency is below the critical frequency, f<fc, • then the photoelectric effect doesn't happen. • When the photoelectric effect does happen • the electron gets more energy from higher frequency light • Classical wave theory of light cannot explain why frequency should have any effect.

  15. PEE – Critical Frequency If light’s frequency is below the critical frequency, f<fc, • then the photoelectric effect doesn't happen • Photon theory (with E=hf) explains: • Low frequency light means low energy photons. • If f<fc, then the photon doesn’t have enough energy to free the electron from the metal.

  16. PEE - Critical Frequency If light’s frequency is below the critical frequency, f<fc, • then the photoelectric effect doesn't happen. • But if f=fc, the energy of the photon is just barely enough to free the electron from the metal. F = energy needed to free electron • For effect to occur hf > F, f > fc. • If f>fc, the effect does occur • The electron gets all the photon’s energy, hf

  17. PEE – Kinetic Energy When the effect does occur, increasing the frequency of the light increases the kinetic energy of the electrons that are released • Photon theory explains: • photon’s energy, hf, is absorbed by electron, • some energy is used to free electron, • rest of energy is kinetic energy.

  18. K f 0 0 PEE – Kinetic Energy The kinetic energy of the ejected electrons increases with the frequency of the light Slope=h fc -F No effect

  19. Test it Yourself http://lectureonline.cl.msu.edu/~mmp/kap28/PhotoEffect/photo.htm

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