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
- 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
- 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
- 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."
- 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.
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Revised October 12, 2007