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Waves, Photons & the EM Spectrum

Waves, Photons & the EM Spectrum.  Astronomers obtain information about the universe mainly via analysis of electromagnetic ( em ) radiation : visible light radio waves x-rays infrared radiation and so on . . . EM radiation sometimes behaves like waves ,

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Waves, Photons & the EM Spectrum

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  1. Waves, Photons & the EM Spectrum

  2.  Astronomers obtain information about the • universe mainly via analysis of electromagnetic • (em) radiation: • visible light • radio waves • x-rays • infrared radiation • and so on . . . • EM radiation sometimes behaves like waves, sometimes like particles!

  3. Waves

  4. WAVES A wave is a moving disturbance. Two kinds of waves in a slinky. The slinky is the wave medium.

  5. [Wave animations]

  6. Words that describe waves . . . crest Wavelength () Amplitude trough Period (T): time for one wave to pass a point Frequency (f): # of waves passing a point per second

  7. Compare two waves:  Long wavelength  Short wavelength Short wavelength  short period, high frequency Long wavelength  long period, low frequency [Animation . . .]

  8. Electromagnetic Waves  Oscillating magnetic and electric fields  Sources: accelerated charge (e.g., vibrating electrons) • Travel through empty space (no medium)  Travel at speed of light (c) in vacuum: c = 300,000 km/sec = 186,500 mi/sec

  9. Electromagnetic Wave Motion Magnetic Field Electric Field

  10. Electromagnetic Spectrum: Span of all em wavelengths Visible: part we can see. p. 101

  11. UV IR  Visible Spectrum “ROY G. BIV” Units: Nanometer (nm): 1 nm = 10-9 meter Ångstrom (Å): 1 nm = 10 Å

  12. Photons • 1900 – 1905: Max Planck & Albert Einstein find light sometimes behaves like particles: photons • Photons carry energy (E): E  Frequency (E  f), or E  1/Wavelength (E  1/)

  13. Long wavelength  Low energy Short wavelength  High energy

  14. Interaction of Light & Matter • Emission • Absorption • Transmission • Reflection

  15. Boy Dog Infrared Continuous emission by a solid

  16. ‘Cool’ ‘Warm’ ‘Hot’

  17. Continuous emission by dense gas (Stars) Cool Warm

  18. Selective emission by a thin gas

  19. Selective emission by a thin gas

  20. white light Selective reflection & absorption by solids

  21. selective reflection & absorption by solids & gases

  22. Spectra I procured a triangular glass prism, to try therewith, the celebrated phenomena of colors. And for that purpose, having darkened my laboratory, and made a small hole in my window shade, to let in a convenient quantity of the sun’s light, I placed my prism at the entrance, that the light might be thereby refracted to the opposite wall. It was at first a very pleasing diversion to view the vivid and intense colors produced thereby. - Isaac Newton

  23. A spectrum is produced whenever light from any source is broken-up into its constituent wavelengths (or frequencies): Spectrum Incoming Light Prism (Disperses light)

  24. Three Types of Spectra • 1.Emission (Bright) Line •  Bright lines on a dark background • 2. Absorption Line •  Dark lines on a bright background • 3. Continuous •  Continuous band of colors

  25. p. 105

  26. Emission Line Spectra H Na He Ne Hg  Note:unique pattern for each element.

  27. Intensity Wavelength Absorption Line Spectra

  28. The Sun’s Spectrum

  29. Emission/Absorption patterns identical! All three kinds of spectra

  30. Wavelength  Hydrogen Energy  p. 102

  31. Continuous Spectra • Spectrum not equally bright (Intense) at each point . . . • Measure intensity at each wavelength, then plot intensity vs wavelength . . .

  32. . . . You get this: red violet

  33. Two rules of black bodies

  34. T4 Amt. of energy emitted from each sq meter A Couple of Rules for Black Bodies • As temp (T) increases, more energy is emitted from • each unit surface area. • As temp (T) increases, the peak of the BB curve shifts • to shorter wavelength.

  35. Orion Compare two stars: Betelgeuse: T  3,000 K Rigel: T  12,000 K

  36. As temp drops, location of peak drifts to longer wavelength. 7000 K Intensity 6000 K 5000 K p. 104 Wavelength

  37. 400 nm 700 nm . . . So the Color Changes “Hot:” Blue “Cold:” Yellow

  38. ‘Cool’ ‘Warm’ ‘Hot’ p. 103

  39. Spectrum of the Planet Mars (Complicated!) p. 106

  40. The Doppler Effect The Doppler Effect: Change in observed wavelength and frequency of waves due to radial motion of source and/or observer.

  41. Wave crests Observer Source No source motion: no change in f or λ

  42. Motion toward observer: f increases & λ decreases Motion away from observer: f decreases & λ increases. No change in f & λ here! Doppler animations . . . p. 100

  43. Astronomically speaking . . . . . . For a star moving toward/away from Earth . . .

  44. . . . We find a shift in the absorption (or emission) lines: Star moving toward Earth  lines shifted toward shorter wavelength: Blueshift

  45. Star moving away from Earth  lines shifted toward longer wavelength: Redshift

  46. In either case, velocity  amt ofwavelength shift

  47. v Galaxy spectra – all redshifted 1200 km/s 15,000 km/s • Larger shift • Larger Velocity 39,000 km/s 61,000 km/s 

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