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Prospective Learning Assistant meeting Wed, Oct 17, 6 PM, with food.

Prospective Learning Assistant meeting Wed, Oct 17, 6 PM, with food. RSVP by Oct 10 to Steven.Pollock@Colorado.EDU Why you should go: - if you have any kind of interest in teaching - it's valued by the department

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Prospective Learning Assistant meeting Wed, Oct 17, 6 PM, with food.

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  1. Prospective Learning Assistant meeting Wed, Oct 17, 6 PM, with food. RSVP by Oct 10 to Steven.Pollock@Colorado.EDU Why you should go: - if you have any kind of interest in teaching - it's valued by the department - LAs almost universally consider it to be a great learning experience and a lot of fun. - $1500 you earn for 10 hrs/ week You need to be a strong student - generally an A or strong B, though exceptions can be made for people *dying* to become teachers.

  2. On this week’s homework I spent: • 1 hour or less • 1-2 hours • 2-3 hours • 3-4 hours • more than 4 hours

  3. E b a (a)=hf of the photons c EF d e- e- e- x metal vacuum Which arrow represents the KEmax of an electron ejected from a metal in a photoemission experiment?

  4. E b a A=hf of the photons c EF d e- e- e- x metal vacuum Which arrow represents the KEmax of an electron ejected from a metal in a photoemission experiment? (b) Is correct. KE can be less for electrons below the Fermi energy. KEmax is for electrons initially at EF. (c) = work function f (a) = hf

  5. Night vision goggles use a photocathode with a • Large work-function • Low work-function

  6. Night vision goggles use a photocathode with a • Large work-function • Low work-function

  7. The work function f of a metal is 4.2 eV. An electron is 4.6 eV below the Fermi energy EF. Can it be ejected in a photoemission experiment? • No – never • Yes, but only for hf > 4.2 eV • Yes, but only for hf> 4.6 eV • Yes, but only for hf > 8.8 eV

  8. The work function f of a metal is 4.2 eV. An electron is 4.6 eV below the Fermi energy EF. Can it be ejected in a photoemission experiment? • No – never • Yes, but only for hf > 4.2 eV • Yes, but only for hf> 4.6 eV • Yes, but only for hf > 8.8 eV

  9. KE300 V CQ: A photon at 300 nm will kick out an electron with an amount of kinetic energy, KE300. If the wavelength is halved to 150 nm and the photon hits an electron in the metal with same energy as the previous electron, the energy of the electron coming out is a. less than ½ KE300. b. ½ KE300 c. = KE300 d. 2 x KE300 e. more than 2 x KE300

  10. KE300 V CQ: A photon at 300 nm will kick out an electron with an amount of kinetic energy, KE300. If the wavelength is halved and it hits an electron in the metal with same energy as the previous electron, the energy of the electron coming out is e. more than 2 x KE300 KE = photon energy-energy to get out = hf – energy to get out if l is ½ then, f twice as big, Ephot =2hf300 New KEnew= 2hf300- energy to get out Old KE300 =hf300- energy to get out so KEnew is more than twice as big. hf150 Energy hf300 KE300

  11. Consistent descriptions: • Lots of light means … • Big amplitude E/M wave • Made from photons • (mini E/M wave segments) Bright Red Laser When a photon interacts with something (e.g. an electron) all the energy of its wave segment ends up concentrated in one spot. (like a particle) • Until photon interacts with something (e.g. absorbed by an electron), it is a wave. How does the wavelength of the photon wave compare to the wavelength of the light in the red laser? • Photon has a smaller wavelength • Photon has same wavelength • Photon has a larger wavelength Answer is b. Photon has same wavelength

  12. Probability and randomness Photon is 3-D spread out little chunk of EM wave. Gazillions of electrons in metal: Which one will be kicked out? Can’t tell, but photon uniformly spread out so equal probability. What if shape of single photon wave looked like this? Gazillion electrons Which one will be kicked out? Answer: Can’t tell, but probability of photon collapse at particular point (kicking out electron) related to Intensity of wave (Emax2)

  13. How can light behave like a wave (interference etc), but be made up of particles (photons) that seem to hit at random places? 2 slit interference with wave simulation 2 slit interference with laser http://phet.colorado.edu/simulations/schrodinger/schrodinger.jnlp http://phet.colorado.edu/simulations/waveinterference/waveinterference.jnlp

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