Gustav Kirchoff articulated some basic phenomena that occur when
light flows through matter. These rules are fundamental tools when
performing spectroscopy, which is the study of matter from the light
that comes from or through objects.  The laws are straightforward.

 - Light flowing through high-density, low-energy gas produces absorption
lines.  The patterns of the lines reveal the elements or compounds in the gas.

 - Light flowing from low-density, high-energy gas produces emission lines.
The patterns of the lines reveal the elements or compounds in the gas.

 - Light flowing from high-density, high-energy gas produces continuum light.
The shape of the continuum reveals the process that energizes the gas.  If, for
example, the gas has been energized by being heated to a temperature higher than
its surroundings, then the continuum light will be thermal radiation.

Any, all or none of these features can appear in a spectrum.  The combination 
of the spectral information from light sources reveal a great deal about those
sources, and is often the only tool that can be used to learn about much of
our universe.  Spectroscopy is particularly important in astronomy - a science
that depends critically on the light coming from distant objects.