- As opposed to in a solid, in a tenuous gas atoms can
assert their ``individuality''.
- The electrons in an atom can only ``orbit'' the nucleus at
- A planet can orbit the Sun at any distance. Not so with the electrons in an atom. They are restricted to particular ``energy levels."
- Each allowed orbit corresponds to a specific energy.
- An atom emits or absorbs a photon when an
electron moves from one energy level to another.
- The energy (i.e. the wavelength) of the photon depends on
the exact difference in energy between the two levels.
Energy, Wavelength, and Color are essentially the same
- An atom in which an electron jumps from a higher to a lower
energy state emits a photon of the corresponding energy.
- Electrons will naturally descend to the lowest energy state available. Thus an electron displaced to a higher energy level will "fall" back
down to the lowest level (possibly in multiple steps) emitting light of a
specific wavelength at each step.
- An atom can only absorb a photon if that photon has an energy
(i.e. wavelength) exactly equal to the difference in energy between
the present level of an electron and one of the higher levels.
- The available energy levels are unique to each element in the
- Each element (and molecule as well) has a distinct spectral
Spectral Lines are Useful because...
- They reveal the elemental makeup of a gas.
- They can reveal the relative amounts of different elements in a
- The ``strength'' of a line depends on how much of the
corresponding element is present.
- They reveal the temperature of a gas.
- Although an atom is capable of producing a variety of
spectral lines, only certain ones are produced at any given temperature.
- Atoms are "primed" for absorption or emission depending on
where their electrons lie in the energy level structure.
- At low temperatures the electrons are in the lowest
energy levels ("closest" to the nucleus).
- At high temperatures collisions between atoms cause
electrons to be distributed at different levels -- the hotter the
gas the higher the level.
- An example -- The visible "Balmer" lines of Hydrogen gas
originate from electrons dropping down into the 2nd energy level.
- At "low" temperatures all of the electrons are in the
first energy level with no access to the second. No lines.
- At "intermediate" temperatures electrons regularly flirt with the 2nd energy level permitting emission and absorption of the Balmer lines. Strong lines.
- At "high" temperatures electrons can be found throughout
the energy level structure, only rarely visiting the 2nd energy level. Weak lines.
- They reveal the velocity of the gas toward or away from the observer.
- Spectral lines have very precisely known wavelengths.
The observed spectral lines are shifted from their ``rest''
wavelengths depending on the velocity of the source.
Updated October 12, 2007