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Lecture 3

Lecture 3. Electromagnetic Radiation and Quantum Theory 1.2-1.4 27-Aug Assigned HW 1.4 , 1.8, 1.10a, 1.16, 1.20, 1.30, 1.34 Due: Monday 30-Aug. Review B.1-B.4, 1.1. The current model of an atom is, in large, due to the experiments of 4 dudes:

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Lecture 3

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  1. Lecture 3 Electromagnetic Radiation and Quantum Theory 1.2-1.4 27-Aug Assigned HW 1.4, 1.8, 1.10a, 1.16, 1.20, 1.30, 1.34 Due: Monday 30-Aug

  2. Review B.1-B.4, 1.1 • The current model of an atom is, in large, due to the experiments of 4 dudes: • John Dalton hypothesized an Atomic Theory • J.J Thompson discovered the electron with a cathode ray tube • Millikin determined the mass of an electron • e = -1.6022 x 10-19 C • Rutherford used alpha particle diffraction off a thin sheet of metal foil to hypothesize the nucleus • The nucleus is made up of protons and neutrons and accounts for most of the mass of an atom • Electrons surround the nucleus and account for most of the volume of an atom. • Atomic number (Z) is the number of protons • Mass number is total number of protons and neutrons • Isotopes have the same number of protons but vary in neutrons • The periodic table organizes elements according to similar characteristics

  3. Waves Waves transmit energy Water waves Seismic Waves Both differ from electromagnetic radiation since they require material medium for their transmission

  4. Properties of Waves Maximum height of the wave (above the center line) is the Amplitude The distance between the top point of two successive waves is the Wavelength (λ) (units of distance, SI  m) • Frequency (ν) is the number of times a wave passes through a single point in a give amount of time (units  time-1, SI  Hz) • units?

  5. Electromagnetic Radiation • Electromagnetic radiation • Form of energy transmission • Electric and Magnetic fields are propagated as waves through empty space (vacuum) or through a medium (i.e. glass) • Two fields are orthogonal

  6. Electromagnetic Radiation • For electromagnetic radiation, velocity is a constant • Speed of Light (c = 2.998 x 108 ms-1) • Example: Most of the light from a sodium vapor lamp has a wavelength of 589 nm, calculate the frequency.

  7. Electromagnetic Spectrum V I B G Y O R

  8. Electromagnetic Spectrum • Vertebrates have two types of photoreceptors: • Rods - do not require very intense light but can not differentiate color (night vision) • Cones - require intense light but allow color differentiation (daytime) • Wanna know more?

  9. Sunlight and the Continuous spectrum • When sunlight passes through a prism (water), the light is separated by wavelength • Why? • Sunlight  continuous spectrum • Contains many wavelengths • Not all sources of light are continuous

  10. Sodium Spectrum Sodium burns very brightly and emits a yellow color Discontinuous spectrum

  11. Hydrogen vs. the Continuous Spectrum • When hydrogen is passed through a prism, we observe only a few spectral lines • This is an example of a discontinuous spectrum

  12. Hydrogen Spectral Lines • In 1885, Johann Balmer deduced a formula that can predict the visible lines in the Hydrogen Emission Spectrum Does it work? Calculate λ for the n=3.

  13. Hydrogen Spectral Lines • Johannes Rydberg noticed that all spectral lines of hydrogen could be predicted by

  14. The Inception of Quantum Mechanics • The well defined spectral lines suggests that an electron can only exist at specific energy levels • Each energy level has a value of • Why is the frequency of the photon emitted be the difference between the two energy levels, or quanta? • These can NOT be explained by classical physics.

  15. Blackbody Radiation • Important clues came from heating solids to extreme temperatures and monitoring the intensity of the emitted radiation at various temperatures – or blackbody radiation • Classical theory predicts that the intensity of the radiation emitted would increase infinitely

  16. Blackbody Radiation • 1879 – Josef Stefan noted that c = 5.67 x 10-8 Wm-2K-4 • Stefan-Boltzmann law • 1883 – Wilhelm Wien noted that CW = 2.9 K•mm • Wien’s law

  17. Applying Blackbody Radiation Observations • A red giant (almost dead star) shows a wavelength maximum at 700nm. What is the surface temperature?

  18. Max Planck and Quantum Theory • 1900 – a German physicist made a proposal: • Energy, like matter is discontinuous. • When energy increases from one allowed level to another, there is a tiny jump. • Each discreet level has a well defined energy

  19. Photoelectric Effect – the QT test • When light (hν) strikes a metal surface, electrons are ejected • Emission ONLY occurs when the incident light exceeds a threshold (φ) • The number of electrons emitted depends on the intensity of the incident light. • The kinetic energies of emitted electrons depend on the frequency of the light. Energy of Incident light Kinetic Energy Of ejected Electron Energy Required For Ejection Video

  20. Photoelectric Effect – the QT test Example: The wavelength of light needed to eject an electron from hydrogen is 91.2 nm. Calculate the velocity of the particle ejected when 80.0 nm light is used to shone on a sample of hydrogen.

  21. Photoelectric Effect – multiple samples These experiments verify that for each element, discreet amounts of energy are required to eject an electron, φ, which corresponds to a difference between two well defined energy levels

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