1. OUR UNIVERSE Lectures 7 - 9

2. The Physics of Radiation & Spectroscopy The windows to Our Universe & the keys to our knowledge & understanding. The Physics in Astrophysics.

3. Light is electromagnetic radiation Oscillating Electric & Magnetic fields wavelength frequency=c/ E c speed c B

4. To produce electromagnetic radiation we must accelerate electric charge e- Oscillation back-and-forth • Oscillating currents (e-) • in antennae (radio, TV, • radar, microwaves, etc) • in atoms • (IR, visible light, X-rays, etc)

5. To produce electromagnetic radiation we must accelerate electric charge Radio  Gamma rays Low  High energy  km  10-14 m + e- electron Deflected by a nucleus - bremsstrahlung

6. Bending in magnetic field: synchrotron radiation e- also sometimes called magnetic bremsstrahlung

7. C CO Carbon Monoxide +- +- O We can picture a diatomic molecule as a dumbell =

8. To produce electromagnetic radiation we must accelerate electric charge C O Typically  1-100 µm Infrared (IR FIR) Vibrations of a diatomicmolecule

9. To produce electromagnetic radiation we must accelerate electric charge C O Typically  mm  cm mm  microwaves Rotation of a diatomic molecule

10. ro-vibrational spectrum of CO

11. ro-vibrational spectrum of CO

12. ro-vibrational spectrum of CO

13. The Electromagnetic Spectrum

14. The Electromagnetic Spectrum  from Radio  Gamma Rays

15. Radio Gamma Rays X-rays Ultraviolet (UV) Visible Infrared (IR) mm waves Microwaves

16. Atmospheric Windows Atmosphere is transparent 100 nm 1mm 1 m 10 µm 1cm 10 m 1 µm 100 µm Visible: 400-700 nm Radio Window Optical Window Transmission  Wavelength

17. Interference of Waves A consequence of the wave-like nature of radiation is interference & diffraction. Constructive Interference

18. Interference of Waves Destructive Interference

19. Interference of Waves Young’s Experiment: 2-slit interference

20. Interference of Waves Diffraction through a single slit. D Diffraction peak

21. Diffraction through a circular aperture, diameter D. D

22. Diffraction through a telescope of Diameter D: the diffraction-limited angular resolution is:  in radians  in arcsec

23. Images merge as 2 sources moved together to below the angular resolution

24.  in arcsec = 0.063 arcsec What is the diffraction limit for a 2.4m telescope for light with l=600 nm?

25. Electromagnetic Radiation • behaves in 2 complementary ways: • waves - frequency = c/ • particles (photons) - energyE = h • Atoms & molecules emit and absorb • radiation in discrete quanta of energy h • The frequencies are characteristic • of atomic & molecular structure. • (The photons are their “fingerprints” or “DNA”)

26. The Rutherford model of the atom. classical e- (electron) orbits

27. Quantum Mechanics gives discrete “orbits” for the e- in a Hydrogen atom. In each orbit the e- has a discrete energy:

28. H atom: Allowed orbits for thee- Ground state n=1 1st Excited state n=2 2nd Excited state n = 3 3rd Excited state n = 4

29. Emission & Absorption of Radiation • In each orbit the e- has a • unique quantised energy: • In falling down from • orbit m  n a photon of energy • h = Em -Enis emitted. • In jumping up from • orbit n  m a photon of energy • h = Em -Enis absorbed.

30. Absorption & emission of an Hphoton by Hydrogen  = 656 nm

31. Absorption & emission of an Hphoton by Hydrogen  = 656 nm

32. Emission & Absorption of Radiation • In each orbit the e- has a unique quantised energy: • Transitions down(emission) & up (absorption) • from level n give rise to unique, identifiable • spectral lines. • Therefore Spectral lines provide • powerful methods for: • (a) identifying different elements • (b) discovery physical conditions in space

33. Hydrogen atom Spectral Series LL etc HH etc PP etc

36. Hydrogen atom Spectral Series

37. Emission Spectra for rarefied gases & vapours of the elements.

38. Emission Spectra for rarefied vapours of the elements. This example is the Omega nebula, M17

39. M17 H=656 nm

40. The typical reddish pink glow of Hydrogen excited by young stars in the galaxy NGC 2363 (in the constellation Camelopardis)

41. NGC 2363 H=656 nm Hydrogen

42. H=656 nm Hydrogen

43. z = 6.58, 97%c NGC 3310: z = 0.0033 v = 1000 km/s Markarian 609: z = 0.034 v = 10,000 km/s

44. 500 550 600 650 Wavelength  nm Intensity Spectra of the 2 galaxies

45. 500 550 600 650 Wavelength  nm Intensity H H Laboratory wavelengths 0

46. Emission Spectra for rarefied gases & vapours are line spectra, unique for each element; but we also often see an underlying continuum.

47. What causes the continuous spectrum?

48. Kirchoff’s Laws of spectroscopy. 1) A low density hot gas emits discrete lines - emission lines. 2) A hot solid, liquid or dense enough gas emits a continuous spectrum. 3) A cool gas absorbs radiation at the same frequencies as it emits when hot - this produces dark absorption lines.

49. Kirchoff’s Laws of spectroscopy. 1) A low density hot gas emits discrete lines - emission lines. These lines are a unique signature of the atoms in the gas.

50. A low density hot H gas: discrete emission lines.