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AP Physics Day 45 – Photoelectric Effect and Compton Scattering Date 1/8/2004 Overview: HW Check & Review Ch 27: 1,3,4,6-8,11 Photoelectric Effect Activity – Collect Data & Analyze Part A. Notes - Photoelectric Effect & Compton Scattering HW: Ch 27: 15-19,27,33,35,36,37,40-43 Finish Photoelectric Effect Activity Papers: Photoelectric Effect Activity Materials: Demos: *Blackbody Radiation 200 W bulb diffraction grating glass variable transformer Photoelectric Effect Photoelectric Effect Device (w/battery) Oscilloscope Websites Labs: Photoelectric Effect Applet: http://lectureonline.cl.msu.edu/~mmp/kap28/PhotoEffect/photo.htm Notes: Notes – Photoelectric Effect & Compton Scattering Planck wins Nobel Prize for his work in 1918, but that is after the idea receives a lot of support: First, in 1905 the idea of light quanta are used to explain another experimental result: the photoelectric effect. Photoelectric Effect – Light striking the surface of a sheet of a photosensitive metal can knock an electron free from the metal. Continuous light keeps knocking electrons free and you get a current This is simple – the energy from the photon lifts the electron out of the electrical potential energy well that it is in inside an atom and the electron is free. If you shine a brighter light (with more energy) on the material, more electrons are knocked free and you get a bigger current. You can control this by putting a voltage across the plates to work against the electrons. Electrons lose kinetic energy as they travel and will stop if they run out of kinetic energy before they hit the electrode. The higher the voltage the more get stopped until you reach some stopping voltage, V0. Kmax = eV0 Here is data for one particular color of light at two intensities. The more intense light does produce more current and we can turn up the voltage to stop the electrons. Notice that for this color, the stopping voltage is exactly the same, regardless of intensity As you change colors, the cutoff voltage shifts as well. The higher the frequency of the light, the higher the cutoff frequency is! Higher frequency light gives electrons more Kinetic energy! As you decrease the frequency of the light, there comes a point, where the light does not produce a current, no matter what. This frequency is the cutoff frequency (f0), which depends on the metal being used. You get at least some current above cutoff frequency even with very dim light. Einstein explains all of these effects by extending Planck’s quantum idea. He named each of the little bits of light energy photons. Ephoton = Kelectron + Where = PEelectron (work function) hf = Kmax + 0 0 is minimum PE 0 hf0 = 0 + 0 f0 = h Samples http://lectureonline.cl.msu.edu/~mmp/kap28/PhotoEffect/photo.htm Compton Scattering In some materials, a photon hitting an electron is not totally absorbed. Instead the photon hits the electron and they both fly off in different directions, just like two pool balls. The trick is that for the electron to gain KE and fly off, the photon must lose energy – which means that its frequency and wavelength change as well. The amount by which the wavelength changes depends on the angle of the collision, shown by the angle at which the photon exits = f – i h = (1-cos) mec