In the late 19th century, physicist Max Plank
was trying to mathematically model blackbody
emission of light.
Previous attempts had failed to describe the
behavior of blackbody radiation over the entire
spectrum.
It had been assumed that the energies of atoms
behaved much like energies associated with
other familiar systems.
Plank's breakthrough came when he restricted
those energies to discrete values.
Atoms of the same element would have the same
allowed energies, but those values would differ
from the allowed energies of other elements.
Atoms then had to gain or lose energy by the
absorption or emission of packets of energy.
Einstein would show that these packets were
actually individual photons.
Some additional terms are needed for further
discussion.
For simplicity, let's restrict this discussion
to the hydrogen atom.
A hydrogen atom has only one electron.
The energy of an atom is basically dictated
by the location of its electron.
When the electron is in the lowest state possible,
that is, when the atom is in the lowest state
possible, the atom is in what is referred
to as the ground state.
When in the ground state, electrons tend to
be closer to the nucleus.
The nucleus is positively charged and electrons
are negatively charged.
The closer opposite charges are, the greater
the electrical attraction and the more tightly
bound the electrons are to the nucleus.
If electrons have more energy than the ground
state, the atom is in an excited state.
If an electron breaks free, then the atom
has been ionized.
Shown here are the allowed energy levels of
a hydrogen atom.
The unit of energy used when dealing with
atoms is the electron-volt.
The energies are negative, indicating that
the electron is bound to the atom.
The lower the energy, the more tightly bound
the electron is to the nucleus.
The ground state is -13.6 electron-volts.
At energy of zero or greater, the electron
breaks free and the atom is ionized.
Electrons jump instantaneously from one level
to another by the emission or absorption of
one photon per jump.
They can only go from one allowed energy level
to another allowed level.
Thus, only certain photon energy will be absorbed
or emitted.
The photon energy will be equal to the energy
difference between levels.
Photon energies are always positive, so don't
get too hung up on the sign.
The sign just indicates whether a photon is
emitted or absorbed.
When an electron absorbs a photon, it goes
up to a higher energy level, indicated by
the upward arrow.
The electron will spontaneously drop back
down in energy by the emission of a photon,
indicated by the downward arrow.
The energy difference between the ground state
and the first excited state is 10.2 electron
volts.
An electron can jump from the ground state
to that first excited state only by the absorption
of a photon that possesses 10.2 electron-volts
of energy.
If an electron in the ground state absorbs
13.6 electron-volts or more, the electron
breaks free from the nucleus and the atom
is left ionized.
Since electrons will only emit and absorb
photons with certain energies, and since the
energy of a photon is precisely related to
its wavelength, then only certain colors of
the spectrum are emitted or absorbed by a
gas of hydrogen atoms.
Energy levels are labeled here by an index
n.
Suppose one has a gas of excited hydrogen
atoms.
When electrons in those atoms make a transition
from n=3 to n=2, that corresponds to the wavelength
of the red light.
If you look at the spectrum, this would be
the red line on the right.
This is an example of an emission line.
A spectrum of emission lines is an emission
spectrum.
Different energy-level transitions result
in different colored emission lines.
Only a few energy-level transitions emit photons
that are within the visible domain.
Most of the transitions emit photons that
are outside the visible domain.
Under certain conditions, one can have absorption
spectra, as shown below the emission spectrum.
They appear as continuous spectra with dark
bands that correspond to colors of light that
have been absorbed.
These dark bands are called absorption lines.
It is important to note that the wavelengths
of the absorption lines are the same as the
wavelengths of the emission lines in the emission
spectrum.
Collectively, absorption and emission spectra
are called line spectra.
Since each element has its own unique set
of allowed energy levels, they each have their
own unique line spectra.
Some sample emission spectra are shown.
Line spectra are like the fingerprints to
the elements.
For example, by collecting light from a star,
creating a spectrum and analyzing the absorption
lines present, one can determine what elements
are present in that star.
Shown here is a power spectrum.
In power spectra, vertical spikes are emission
lines.
Analysis of line spectra is not limited to
the visible domain.
These are emission lines that are actually
at wavelengths corresponding to x-rays!
These are very energetic photons coming from
near a neutron star.
Computers have been used to identify the elements
in the environment of the neutron star.
Those atoms are being very energetically excited
and when they de-excite, they emit high-energy
photons.
When an atom is bound up as part of a molecule,
this adds additional allowed energy levels
due to additional rotational and vibrational
energies.
Even if the atoms are of the same kind, the
line spectra become more complex.
Each compound also has its own unique set
of allowed energy levels.
Thus, by analyzing spectra, we can also determine
what compounds are present.
Absorption lines occur when photons from a
continuous light source, like the surface
of a star, passes through a cooler gas cloud.
Photons of certain wavelengths get absorbed
and re-radiated.
However, when these photons are re-radiated,
they usually do so in different directions.
This leave the dark absorption lines in the
spectrum.
On the top left is an absorption spectrum
and its corresponding power spectrum just
to the right.
It's basically a blackbody curve with absorption
lines appear as downward spikes.
On the bottom left are emission lines and
its corresponding power spectrum.
The emission lines appear as upward spikes.
You should be able to identify a spectrum
as either an emission or absorption spectrum.
