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Modern Physics

Modern Physics. Wave - Particle Duality HW: Read pages 723 – 734 Answer questions p 730 #1,2,3,4 P732 # 6, 7, 8. How do we see objects?. Blackbody Radiation. Warm objects emit infrared radiation. Objects at ~1700 K, glow red. Above 2000 K, glow white. As temp increases, EM

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Modern Physics

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  1. Modern Physics Wave - Particle Duality HW: Read pages 723 – 734 Answer questions p 730 #1,2,3,4 P732 # 6, 7, 8

  2. How do we see objects?

  3. Blackbody Radiation • Warm objects emit infrared radiation • Objects at ~1700 K, • glow red • Above 2000 K, glow white • As temp increases, EM • radiation emitted at higher freq

  4. Planck’s Explanation • Energy emitted was not continuous • Consists of finite # of small discrete amts related to freq of radiation • E =n h f where h = constant • f = frequency • n = positive integer • Energy that can only have definite values is said to be quantized

  5. Planck’s Explanation (cont’d) According to Conservation of Energy, the energy of emitted EM waves must be equal to the difference of the oscillation energies of the atoms E = hf final - hf initial h is the slope of the graph of photon energy vs photon freqeuncy h = 6.63 x 10-34 J s

  6. Photoelectric effect • Light shines on metal surface • Electrons will only be emitted from a metal surface if light has sufficient energy • E = hf OR • E = hc l

  7. The electron Volt (eV) • The Joule is too large of a unit to work with for atom sized systems • The electron volt is the energy needed by an electron to accelerate across 1 Volt of potential difference 1 eV = 1.6 x 10-19 J In terms of eV, Ephoton = 1240 eV nm l

  8. Photoelectric effectEinstein’s proposal Light must be emitted as packets of energy (quanta or photons) Light must be transmitted as tiny particles (photons) Photons of light shining on metal is absorbed by electrons

  9. Einstein’s proposal cont’d • In order for electrons to be ejected from atoms: • the photons they absorb must have a minimum amount of energy (work function) based on their frequency (called threshold) f0 = threshold freq

  10. KE of ejected electrons • Electrons that absorb photons with more energy than they need to be ejected from an atom move away from the atom with KE • The KE of the electron is equal to the difference between the energy of the photon and the work function of the electron: KE = Ephoton – Work function KE = hfphoton - hf0

  11. The Compton Effect • X-ray scattering proved that photons have momentum Momentumphoton = hf/c Pphoton = h/l • Secured the idea that light acts as a particle

  12. de Broglie All matter must exhibit wave like properties A beam of electrons diffracts like light

  13. NeilsBohr • Energy emitted from a blackbody: • Ephoton = Ei – Ef

  14. Question: • How much energy does a photon have that is released by an electron of hydrogen as it fall from the third energy level to the first? • What is the frequency of this photon? • What type of electromagnetic wave is this photon ?

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