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Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
Lecture19221
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Lecture19221
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Lecture19221
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Lecture19221

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a supplemental resource for students

a supplemental resource for students

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  • 1. Quantum Theory: the Nature of Light and Atomic Spectra Lecture 19
  • 2. 1890-1930: revolution in science.
  • 3. Rutherford atomic model
  • 4. A nucleus and an electron must attract each other, so why do they remain apart?
  • 5. Well, they are apart because the electron moves. Kinetic energy of the electron’s motion balances the potential energy of nucleus’ attraction.
  • 6. However, a moving electron must emit radiation, thus lose energy, approach the nucleus, and annihilate with protons! e - + p + = γ + γ
  • 7. But it does not happen. The theory was imperfect. 1890-1930: revolution in science!
  • 8. Energy and matter are distinct in macroscopic world. However, this distinction fades in microscopic world. Just rethink…
  • 9. Light is both energy and matter, both waves and particles.
  • 10. Light is a type of electromagnetic radiation. Electromagnetic radiation consists of energy propagated by electric and magnetic fields.
  • 11. Light as a wave. Electromagnetic wave is formed by electric and magnetic fields that alternately decrease and increase in intensity, moving through space.
  • 12. Any wave is characterized by:
    • Frequency – the number of cycles the wave undergoes per second.
    • Wavelength – the distance between any point on a wave and the corresponding point on the next crest (or trough) of the wave.
    • Amplitude – the height of the crest (or the depth of the trough) of each wave.
    • Speed – the distance the wave travels per unit time.
  • 13. Electromagnetic radiation everywhere.
  • 14. In case of light, we perceive frequency (and wavelength) as color, amplitude as brightness.
  • 15. Regions of electromagnetic spectrum
  • 16. Monochromatic light is that of a single wavelength. It is like a tone in music.
  • 17. Polychromatic light is that of many wavelengths. White light is polychromatic.
  • 18. Regions of electromagnetic spectrum
  • 19. Regions of electromagnetic spectrum
  • 20. All the electromagnetic waves travel at the same speed through a vacuum (300,000 km/s) but differ in frequency and wavelength.
  • 21. A sample problem on interconverting wavelength and frequency.
  • 22. Always use c = λ * ν
  • 23. Waves undergo refraction
  • 24. Waves undergo diffraction
  • 25. Waves undergo interference
  • 26. So at the end of 19 th century all everyday and laboratory experience seemed to confirm the classical distinctions between the wave nature of energy and the particle nature of matter .
  • 27. Light as particles.
  • 28. What confounded physicists in the 20 th century :
    • Blackbody radiation;
    • The photoelectric effect;
    • Atomic spectra.
  • 29. The black body problem
  • 30.
    • Max Karl Ernst Ludwig Planck (1858-1947), German scientist
  • 31. An object can emit (or absorb) only certain quantities of energy: E=nhν
  • 32. If an atom can emit only certain quantities of energy, then the atom itself can have only certain quantities of energy. The energy is quantized!
  • 33. Quantum is an energy packet, a “fixed quantity” of energy. The quantum’s energy equals hν.
  • 34. An atom changes its energy state by emitting (or absorbing) one or more quanta. The energy of the emitted (absorbed) radiation is equal ΔE atom = E emitted(absorbed) radiation = Δnhν
  • 35.  
  • 36.
    • Albert Einstein (1879-1955), German-born scientist
  • 37. Photoelectric effect
  • 38. A sample problem on interconverting wavelength and frequency.
  • 39. Always use c = λ * ν, E = h * ν E = h * c / λ E = 6.626 x 10 -34 J*s – Planck’s constant
  • 40. Light dispersion
  • 41. Spectra of hydrogen
  • 42. Why are these spectra line not continuous like rainbow? Empirically, Rydberg came out with an equation to predict the position of any line:
  • 43.
    • Johannes Robert Rydberg (1854-1919), Swedish scientist
  • 44.
    • Niels Bohr (1885-1962), Danish scientist
  • 45. The Bohr model of the hydrogen atom:
    • The H atom has only certain allowable energy levels – stationary states.
    • The atom does not radiate energy while in one of its stationary states.
    • The atom changes to another stationary state (the electron moves to another orbit) by absorbing or emitting a photon whose energy equals the difference in energy between the two states.
  • 46. Bohr atomic model
  • 47. The Bohr explanation of spectral lines
  • 48. The key point: the energy of an atom occurs in discrete levels.
  • 49. THE END

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