2. Waves, Photons & the EM Spectrum

3.  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!

4. Waves

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

6. [Wave animations]

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

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

9. 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

10. Electromagnetic Wave Motion Magnetic Field Electric Field

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

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

13. 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/ )

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

15. Interaction of Light & Matter 1.Emission 2.Absorption 3.Transmission 4.Reflection

16. Continuous emission by a solid Boy Dog Infrared

17. ‘Cool’ ‘Warm’‘Hot’

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

19. Selective emission by a thin gas

21. white light Selective reflection & absorption by solids

22. selective reflection & absorption by solids & gases

23. 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

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

25. 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

26. p. 105

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

28. Intensity Wavelength Absorption Line Spectra

29. The Sun’s Spectrum

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

31. Energy  Wavelength  Hydrogen p. 102

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

33. ... You get this: violet red

34. Two rules of black bodies

35. 1.As temp (T) increases, more energy is emitted from each unit surface area. Amt. of energy emitted from each sq meter  T4T4 2.As temp (T) increases, the peak of the BB curve shifts to shorter wavelength. A Couple of Rules for Black Bodies

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

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

38. 400 nm700 nm “Hot:” Blue “Cold:” Yellow... So the Color Changes

39. ‘Cool’ ‘Warm’‘Hot’ p. 103

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

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

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

43. 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

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

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

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

47. In either case, velocity  amt of wavelength shift

48. v Galaxy spectra – all redshifted Larger shift  Larger Velocity 1200 km/s 15,000 km/s 39,000 km/s 61,000 km/s