1. ATTENTION: EXAM next FRIDAY (one week)!! * Exam covers the reading Chapters 1-6 * Sample questions on the web. HW – due Wednesday midnight

2. Atom-photon interactions Photons:

3. Atom Light Energy exchanged quantized Most commonly by 1 photon “photons”

4. Atom Light Absorption

5. Atom Light Emission

6. Absorption Ground state Atoms in ground state initially

7. Absorption Ground state Have to have a photon present of the right frequency ( n =1 → 2) One photon absorbed

8. Absorption Ground state One photon absorbed Have to have a photon present of the right frequency ( n =1 → 4) Another possibility …

9. White light H atoms slit Absorption spectrum Dark lines appear in spectrum at frequencies (energies) where photons can be absorbed by the atom

10. Emission Ground state Atom must be in excited state

11. Emission Ground state One photon emitted Have to have an atom present in the n = 4 excited state ( n = 4 → 2)

12. Emission Ground state One photon emitted Have to have an atom present in the n = 4 excited state ( n = 4 → 1) Another possibility …

13. H atoms “Hot” slit emission spectrum Have to Jazz the atoms up somehow into their excited states Bright spectral lines appear only at frequencies (energies) that the atoms can emit

14. (Kirchhoff’s Law’s 2 and 3) A diffuse gas emits or absorbs light only at discrete frequencies specific to the substance of the gas

15. Goofy-glasses demo: emission and absorption spectra

16. Matter in general More degrees of freedom more energy levels Simple atomic Complicated molecule lines‘bands’

17. “white light” A white object doesn’t absorb anything Reflective colors WALL

18. “white light” A red object ‘looks’ Red because it absorbs The blue and greens Reflective colors WALL

19. Why green leaves turn red Chlorophyll dies/reabsorbed in autumn leaving longer-lived Carotenoids

20. If an object can emit light at a certain wavelength, then it can also absorb light at that same wavelength! Thermal radiation from continuous sources Recall …

21. Cavity radiation T

22. T

23. T

24. T

25. The ‘color’ and brightness of the light you see in the cavity depends on the relative intensities of the different wavelengths of light present. These intensities depend on the temperature of the walls! For cavity radiation, we can work out the distribution of intensities of all wavelengths, given the temperature T

26. Colored objects …. yellow bodyred body

27. Colored objects …. yellow bodyred body Absorbs: Red, Blue, Green Reflects: yellow Absorbs: Blue & Green Reflects: Red

28. An ideal black body Black body Absorbs light at ALL wavelengths (not just visible!) So, it must be able to emit light at any wavelength too!

29. It turns out that ….. T

30. T Intensity of light of a given  emitted from a unit area of the blackbody

31. It turns out that ….. T Intensity of light of a given  emitted from a unit area of the blackbody

32. IDEAL Thermal-radiation spectra:

33. Most large objects are very nearly like ideal black bodies, with an associated thermal emission spectrum.

34. Cool object (most intensity in IR)

35. The higher an object’s temperature …. The more intensely it emits electromagnetic radiation, and … The shorter the wavelength of the light is at which it’s emitting most strongly!

36. Wien’s displacement law: in °K in m

37. Stefan-Boltzmann law in °K Total power radiated (at all wavelengths) per unit area of the emitting body. in Watts/m 2 5.67×10 -8 W/(m 2 °K 4 )

38. Doppler Effect the water-wave analogy

39. drooling duck

46. rest

47. drooling duck

53. forward

54. drooling duck forward rest

55. source in front in back

56. Doppler effect with light: similar behavior but for very different reasons!

57. source moving away source moving toward REDBLUE

58. RED BLUE

59. Doppler shift measures Radial velocity

60. Transverse velocity

61. Radial velocity Transverse velocity