Radiation: Atoms
and Spectroscopy

Radiation from Different Types of Sources (Kirchhoff's Laws)

  • How do we analyze it? We can separate wavelengths with a spectroscope.
  • Low-density hot gas: Produces an emission line spectrum, atoms emit individually at specific wavelengths.
  • Low-density cooler gas: Produces an absorption spectrum by removing light from continuum.
  • Hot solid or dense fluid: Produces a continuous spectrum, including all wavelengths.
  • Individual particles: If they are moving in a magnetic field, they also produce a continuous spectrum, but it does not look like the ones from dense hot matter; it is not "thermal".

Continuous Spectra: Temperature Effects

  • What we see: A hot solid or dense fluid produces a continuous spectrum, including all wavelengths; also called "thermal spectrum".
  • Heat and temperature: Heat corresponds to the motion energy of atoms or molecules. The lowest possible temperature is absolute zero, 0 K = -273°C = -460°F.
  • Temperature effects: When T increases, the radiation emitted increases at all wavelengths [Stefan's law], and the frequency at which most of the radiation is emitted also increases [Wien's law].
  • Conclusion: Get T from an object's peak emission frequency or wavelength! Once you know T, find its size from its brightness.

  Line Spectra: How Can We Explain Them?

  • What we see: A hot low-density gas produces an emission line spectrum; atoms emit individually, and light comes out only at specific wavelengths.
  • Atomic structure: Nucleus (protons and neutrons), electrons; energy levels, different for different elements.
  • What is going on? Individual atoms and ions absorb/emit only at specific, characteristic wavelengths because fixed energy packets (photons) are absorbed/emitted when an electron jumps between energy levels, and an atom is excited/de-excited.
  • Conclusion: Light comes out of atoms in packets. We also have evidence that it acts as if made of individual particles or quanta when hitting matter [photoelectric effect]. So, photons = both particles and waves; their particle and wave properties are related by Energy = h (frequency).

Line Spectra: What Additional Factors Affect Them?

  • Rotation effect: Lines are spread out as a result of different velocities on opposite sides.
  • Temperature effect 3: Width of spectral lines increased by random motion.
  • Conclusion: If you know T, the line width tells you the rotation rate!

Other Types of Radiation; Conclusion

  • Radio waves from H atoms: When the direction of the electron's spin changes; wavelength l = 21 cm; detected from all interstellar gas which is not too cold [it takes an atom roughly 11 Myr to emit one such photon, but there are lots of H atoms!].
  • Radio waves from molecules: Atoms in them can be in different rotation and vibration states; changes between states usually involves less energy (infrared, radio).
  • Conclusion: Using spectroscopy, we can get information on kind of source, radial velocity, composition, (including ions and molecules) and temperature, from positions and intensities of lines [and rotation from shape of line]; Examples from the Sun, and other stars.

page by luca bombelli <bombelli at olemiss.edu>, modified 29 sep 2012