Hi Ben,
The emission spectrum of a material is the set of frequencies (and
relative intensities of light emitted at those frequencies) of light
given off by the material. Some materials give off light of only a few
specific frequencies when they are excited in specific ways. For
example, if an electric current is passed through a vapor of mecury
atoms, only the frequencies which correspond to transitions between
various configurations of the electrons around the orbit of the mercury
atoms appear. Sodium atoms under similar circumstances give off a
different spectrum, which is why their light is yellow and mercury
vapor lights are more whitish (actually they're a little green). Neon
gives off a nice orangey light. Hydrogen is the simplest case to
calculate from first principles, since it has only one electron and one
proton. It is the emission spectrum of hydrogen which gave physicists
in the early 20th century some of the most important clues about the
quantum mechanical behavior of electrons in atoms.
Most objects consist of large collections of atoms stuck together
in some way. In such large collections, there are many possible energy
states, not just the few specific ones that are allowed to the
electrons in single atoms all by themselves. If a large collection of
atoms is heated up, the spectrum of light it emits is continuous, and
is called the "blackbody radiation law." A real spectrum may have both
kinds of components -- blackbody and discrete emission by atoms. By
measuring how much light is at each frequency, the temperature of the
emitting object may be deduced (this is often how the temperature of
glowing molten steel in steel mills is ascertained -- thermometers
would melt).
Absorption is just emission in reverse. Atoms can absorb light at
the same frequencies they emit at, going from their lowest-energy state
to an excited state. If they re-emit right away at the same frequency,
it's called scattering. If they emit light at some other frequency,
it's called fluoresence.
The sun is very hot and emits lots of light at all frequencies,
mostly following the blackbody curve. If some cooler material is at the
surface of the sun or above it isn't ionized (that is, the atoms still
have their electrons orbiting their nuclei), then these atoms may
absorb some of the light emitted by the sun, but only at the
frequencies which correspond to electronic transitions. Early observers
of the sun used prisms and diffraction gratings to find narrow bands of
absorption in the solar spectrum, and lined these up with the similar
bands observed in the laboratory, and thus deduced the chemical
composition of the outer layers of the sun. Similar investigations have
shown us much about the chemical composition of faraway stars and
interstellar gas and dust. Doppler shifting of these spectral lines
also tells us about the speed of astronomical objects along the
direction of sight.
Tom
(published on 10/22/2007)
