How EM waves are made
To understand what light and other EM waves can tell us, we have to
understand how they are made.
All matter is made of atoms which include a central nucleus surrounded
by electrons in energy levels. It is electrons that are involved with EM
waves. If energy is applied to an electron around an atom, this electron
will absorb the energy, and it will move out of the energy level that it
is in normally, its ground state, to a higher energy level, the
excited
state. Sooner or later, this excited electron will decay back to its
ground state. As it decays it will give off the difference in energy as
a pulse of EM radiation, a photon of a certain frequency. Any time
an atom absorbs any kind of energy ( be it heat, mechanical, or electromagnetic)
that atom will emit EM radiation.
If atoms are in contact with each other in a dense material, solid,
liquid or gas, the atoms will vibrate and collide. The electrons, being
the same charge and repelling each other will be jostled out of their energy
levels and as they return, will give off all different frequencies of EM
radiation. Such a dense object will emit all frequencies, a continuous
spectrum
If atoms are separate from each other, the electrons can only move between
energy levels within the same atom. Because each atom, ion or molecule
has a finite number of electrons, there are only a finite number of frequencies
that are given off when the electrons drop to lower energy levels. This
spectrum is a bright line spectrum. Both continuous and bright line
spectra are created when atoms give off EM radiation and are called emission
spectra.
Atoms can also absorb EM energy. If a continuous spectrum shines through
a non-dense gas, this gas will absorb the same frequencies it would give
off if it emitted. So the continuous spectrum would show these particular
frequencies missing, as black gaps in the continuous spectrum. Such a spectrum
is called an absorption spectrum or dark line spectrum.
What EM waves tell us about their emitter
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First, we can tell density. A continuous spectrum indicates that
the material is either a solid, liquid or dense gas. A line spectrum tells
you you are looking at a thin, non-dense gas.
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Second, we can tell composition. Line spectra given off by thin
gases are unique to each material that gives them off. They are like a
fingerprint. So if we acquire a spectrum from a star, we can compare the
spectral lines from the star to those of known substances, determine what
they are and tell what that star is made of.
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Third we can tell temperature. In spectra, not all frequencies will
be emitted with the same amount of energy. In a continuous spectrum there
will be one frequency at which MOST of the energy is emitted. That frequency
is called peak emission, and peak emission is a function of temperature.
(This can be seen with the coils of an electric stove. As the temperature
of the coils increase, they first emit infrared, then red, then orange.
As their temperature increases their peak emission shifts towards shorter
wavelengths. Conversely, as they cool, their peak emission shifts to longer
wavelengths.)**
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Last, we can figure out relative motion. When an emitter and receiver
are coming toward each other, the apparent wavelength of the emitter decreases
(with a corresponding increase of F), called a blue shift. On the other
hand, when emitter and source move apart, EM wavelengths are stretched,
called a red shift. From the amount of shift, we can determine the rate
of relative velocity: how fast an emitter and a receiver are approaching
or moving apart.
** There are other sophisticated techniques used for line spectra including
strength and presence of particular lines of particular elements. |