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
  • 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. 
  • 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.
  • 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.)**
  • 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.