Spectral Emission, Absorption, and Kirchhoff's Laws

  • An atom can absorb a photon if that photon has an energy/wavelength/color that exactly matches the energy difference between an available energy level and one currently occupied by an electron.

    • Photons of the wrong energy/wavelength/color will be ignored by the atom and pass by.

    • Photons with an energy corresponding to a difference in energy levels between two energy states within an atom will also be ignored if no electron is present in the lower of the two energy states.

  • An atom can emit a photon if an electron drops from a higher energy state to a lower energy state within the atom.

    • The energy/wavelength/color of the emitted photon will be exactly equal to the difference in energy levels.

    • The electrons in undisturbed atoms will reside in the lowest available energy state. Such atom will not produce spectral line emission in this undisturbed state.

    • The absorption of a photon or a collision between two atoms can place an electron in a higher energy state, permitting the production of emission lines.


  • Kirchhoff's Laws address the fact that most sources of light can be approximated as some combination of blackbody radiation and spectral line radiation.

    1. Hot solid objects glow with a continuous spectrum (i.e. pure blackbody radiation).

      • The "color" of the emitted light is simply related to the objects temperature.

    2. A hot tenuous gas produces light at discrete wavelengths (i.e. specific colors) which depend on the energy level structure of the particular atoms constituting the gas.

      • Each atom of the periodic table has a unique "spectral fingerprint."

    3. A hot solid object surrounded by a cool tenuous gas (i.e. cooler than the hot object) will produce a continuous spectrum with light removed (actually redirected) at discrete wavelengths corresponding to the energy level structure of the particular atoms constituting the gas.

      • The Sun (and other stars) are essentially hot blackbodies surrounded by cool gas.

      • Their spectra mainly show dark absorption lines.


Notes


* * * Revised October 12, 2007