Chapter 2 (My Take) It’s what we see (or don’t see for that matter) that counts

1. Chapter 2 (My Take) • It’s what we see • (or don’t see for that matter) • that counts

2. Astro 101 • (including the scientific method) • in a nutshell….

3. Chapter 2Light and Matter

5. Units of Chapter 2 Information from the Skies Waves in What? The Electromagnetic Spectrum Thermal Radiation Spectroscopy The Formation of Spectral Lines The Doppler Effect Summary of Chapter 2

6. 2.1 Information from the Skies Electromagnetic radiation: Transmission of energy through space without physical connection through varying electric and magnetic fields

7. Example: Light

8. Wave motion: Transmission of energy without the physical transport of material

9. Example: Water wave Water just moves up and down. Wave travels and can transmit energy.

10. Frequency: Number of wave crests that pass a given point per second Period: Time between passage of successive crests Relationship: Period = 1 / Frequency

11. Wavelength: Distance between successive crests Velocity: Speed at which crests move Relationship: Velocity = Wavelength / Period

12. 2.2 Waves in What? Diffraction: The bending of a wave around an obstacle Interference: The sum of two waves; may be larger or smaller than the original waves

13. Water waves, sound waves, and so on, travel in a medium (water, air, …). Electromagnetic waves need no medium. Created by accelerating charged particles

14. Magnetic and electric fields are inextricably intertwined. A magnetic field, such as the Earth’s shown here, exerts a force on a moving charged particle.

15. Electromagnetic waves: Oscillating electric and magnetic fields; changing electric field creates magnetic field, and vice versa

16. 2.3 The Electromagnetic Spectrum Different colors of light are distinguished by their frequency and wavelength. The visible spectrum is only a small part of the total electromagnetic spectrum.

18. Different parts of the full electromagnetic spectrum have different names, but there is no limit on possible wavelengths.

20. Note that the atmosphere is only transparent at a few wavelengths – the visible, the near infrared, and the part of the radio spectrum with frequencies higher than the AM band. This means that our atmosphere is absorbing a lot of the electromagnetic radiation impinging on it, and also that astronomy at other wavelengths must be done above the atmosphere. Also note that the horizontal scale is logarithmic – each tick is a factor of 10 smaller or larger than the next one. This allows the display of the longest and shortest wavelengths on the same plot.

21. 2.4 Thermal Radiation Blackbody spectrum: Radiation emitted by an object depending only on its temperature

22. More Precisely 2-1: The Kelvin Temperature Scale • Kelvin temperature scale: • All thermal motion ceases at 0 K. • Water freezes at 273 K and boils at 373 K.

23. Radiation laws: 1. Peak wavelength is inversely proportional to temperature.

24. Radiation laws: 2. Total energy emitted is proportional to fourth power of temperature.

25. Compared to optical photons Radio photons have longer wavelength. X-ray photons have a larger frequency. Infrared photons have a smaller energy. All of the above. None of the above.

26. Which of the following has the LONGEST wavelength? A. Red light. B. Blue light. C. Green light. D. Infrared light.

27. If Star A is hotter thanStar B, and Star A isemitting mostof its light at a wavelengthcorresponding to yellow light,which of the following statements istrue? • Star B will emit most of its light at a wavelength longerthan yellow. • Star B will emit most of its light at a wavelength shorterthan yellow. • Star B will emit most of its light at the same wavelength as Star A. • More information is required to answer this question

28. The energy of a photon is • proportional to the wavelengthand inversely proportionalto the frequency. • proportional to thewavelength and proportional to thefrequency. • inversely proportional to thewavelength and inverselyproportional to the frequency. • inversely proportional to the wavelength and proportionalto the frequency.

29. Electromagnetic radiation will be created by any charged particle that accelerates (changes speed or direction). moves in a straight line at constant speed. remains at rest. is subjected to a gravitational field.

30. 2.5 Spectroscopy Spectroscope: Splits light into component colors

31. Emission lines: Single frequencies emitted by particular atoms

32. Emission spectrum can be used to identify elements.

33. Absorption spectrum: If a continuous spectrum passes through a cool gas, atoms of the gas will absorb the same frequencies they emit.

34. Absorption spectrum of the Sun

35. Kirchhoff’s laws: • Luminous solid, liquid, or dense gas produces continuous spectrum. • Low-density hot gas produces emission spectrum. • Continuous spectrum incident on cool, thin gas produces absorption spectrum.

36. Kirchhoff’s laws illustrated

37. 2.6 The Formation of Spectral Lines Existence of spectral lines required new model of atom, so that only certain amounts of energy could be emitted or absorbed. Bohr model had certain allowed orbits for electron.

38. Emission energies correspond to energy differences between allowed levels. Modern model has electron “cloud” rather than orbit.

39. Atomic excitation leads to emission. (a) Direct decay (b) Cascade

40. Absorption spectrum: Created when atoms absorb photons of right energy for excitation Multielectron atoms: Much more complicated spectra, many more possible states Ionization changes energy levels.

41. Molecular spectra are much more complex than atomic spectra, even for hydrogen. (a) Molecular hydrogen (b) Atomic hydrogen

42. Spectral lines unique to each type of atom are caused by each atom having a unique set of protons. the unique sets of electron orbits. the neutron-electron interaction being unique for each atom. each type of photon emitted by the atom being unique. none of the above; spectral lines are not unique to each type of atom.

43. If an electron moves from a lower energy level to the next higher energy level, then the atom has become excited. the atom has become ionized. the atom's light will be blue shifted. the atom's light will be red shifted.

44. In general, the observed spectra of stars appear as what kind of spectrum? • Absorption. • Continuous. • Emission. • Nonthermal.

45. If you view the light from an opaque, hot gas through a spectroscope, you would expect to see what kind of spectrum? Continuous. Emission line. Absorption line. Combination of emission line and continuous.

46. When an atom is excited, then it has: more electrons than protons. the same number of electrons as protons. one or more electrons stripped off. one or more electrons move to higher energy levels.

47. When you see a spectrum with absorption lines in it, you can infer that: the light passed through ionized atoms. electrons moved up in energy levels to absorb the light. electrons moved down in energy levels to absorb the light. all the atoms were in excited states.

48. 2.7 The Doppler Effect If one is moving toward a source of radiation, the wavelengths seem shorter; if moving away, they seem longer. Relationship between frequency and speed:

49. Depends only on the relative motion of source and observer

50. The Doppler effect shifts an object’s entire spectrum either toward the red or toward the blue.