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PH 301

PH 301. Dr. Cecilia Vogel Lecture 8. Review. Discuss order of events Review Light as a wave (rev PH 202). Photon Three pieces of evidence: blackbody radiation photoelectric effect Compton scattering. Outline. Photon. What is it? Individual, indivisible bit of light energy

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PH 301

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  1. PH 301 Dr. Cecilia Vogel Lecture 8

  2. Review • Discuss order of events • Review • Light as a wave (rev PH 202) • Photon • Three pieces of evidence: • blackbody radiation • photoelectric effect • Compton scattering Outline

  3. Photon • What is it? • Individual, indivisible bit of light energy • How much energy? • For light of frequency, f, each photon has an energy, E = hf • where h = 6.626X10-34 Js Plank’s constant • hc = 1240 eV nm = 1240 MeV fm

  4. Familiar Thermal Radiation • no visible • infrared/heat • Warm 37oC • Red hot 3000K Power spectrum • lots infrared • some red • White hot 6000K • all colors • IR & UV

  5. Peak wavelength The peak in the power graph occurs at shorter wavelength for higher temps. At high temps, its not so hard to muster a lot of energy to produce shorter l’s.

  6. Example a) Find the wavelength of peak emission of the human body (always use Kelvin temperature, here T=310K). b) Is it visible? infrared? UV? c) How much energy does our body lose with each photon it emits at this wavelength?

  7. 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.

  8. -ELECTRIC PHOTO- Photoelectric Effect • Light strikes a metal, knocking electrons off the metal 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...

  9. Photoelectric Effect Detail #1 If light’s frequency is below a critical frequency, f<fc, • then the photoelectric effect doesn't happen • Classical wave theory of light cannot explain why frequency should affect it. • Photon theory (with E=hf)can: • Low frequency means low energy. • If f<fc, then the energy of the photon is too low to free the electron from the metal. F = energy needed to free electron • For effect to occur hf > F

  10. Photoelectric Effect Detail #2 When the effect does occur, increasing the frequency of the light increases the kinetic energy of the electrons that are released • Again classical wave theory cannot explain why frequency should matter. • Only intensity predicted to affect energy. • Again photon theory can: • photon’s energy, hf, is absorbed by electron, • some energy is used to free electron, • rest of energy is kinetic energy.

  11. K f 0 0 Photoelectric Effect Detail #2 The kinetic energy of the ejected electrons increases with the frequency of the light Slope=h fc -F No effect

  12. http://lectureonline.cl.msu.edu/~mmp/kap28/PhotoEffect/photo.htmhttp://lectureonline.cl.msu.edu/~mmp/kap28/PhotoEffect/photo.htm

  13. Compton Scattering before • A photon collides • with a “free” electron • Conservation Laws • momentum • energy • works if photon has • Ephoton=hf • pphoton=h/l. p & E 0 & mc2 after p’ & E’ gmv & gmc2

  14. Compton Scattering before • Conservation Laws • Etot same before and after • ptot same before and after • Note: reflected light has lower freq and longer wavelength • because it lost E and p to the electron p & E 0 & mc2 after p’ & E’ q

  15. What Is Light, Anyway? • Or is it a particle? • blackbody radiation • photoelectric effect • Compton scattering • Light shows particle properties • Is light a wave? • It diffracts • It interferes • Has polarization • Light shows wave properties Wave-particle duality: • Light can show wave or particle properties, depending on the experiment.

  16. When do We See Which? Wave-particle duality: • Light can show wave or particle properties, depending on the experiment. • While propagating, light acts as a wave • while interacting, light acts as a particle.

  17. When do We See Which? • Two-slit experiment • Light will propagate through both slits • and waves through slits interfere with each other, but • when it strikes the screen, • it interacts with the screen one photon at a time.

  18. When do We See Which? • Interference • seen if waves are coherent • Diffraction • seen if obstacle/opening about size of wavelength

  19. Why is the sky blue? • The sky is blue, because more blue light is scattered by the air to our eye (than red, yellow, etc). • Blue light is more likely to scatter than red, because red is more likely to diffract instead. • Less diffraction occurs for shorter wavelengths. • Blue light has shorter wavelength, so it diffracts less and scatters more.

  20. Why are the clouds white? • The water droplets are much larger than the wavelength of all visible light • (not just blue/violet) • almost no visible light is diffracted by clouds • every color of visible light is scattered by clouds • all colors scattered, so scattered light is white

  21. Photon PAL Suppose infrared light of wavelength 1100 nm falls on a material. If the light is very intense, two photons might be absorbed at once by an atom. If that atom then releases all that energy as one photon, this is called two-photon fluorescence. • How much energy does each IR photon have? • How much energy does the atom absorb? • How much energy does the atom release? • What is the wavelength of the emitted light? • What color/type of light is this? • h = 6.626X10-34 Js Plank’s constant • hc = 1240 eV nm = 1240 MeV fm

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