Emission lines are produced when an electron in an excited state jumps down to the ground state and gives off its excess energy in the form of light. Electrons can go to an excited state in the first place either by absorbing a photon or by taking energy from another atom after a violent collision. The energy it gives off is the difference between the energies of the excited state it was in and the ground state. Therefore the emitted light has a single frequency (or wavelength) corresponding to that energy. The emitted light has the form of a photon (squigly line below right). In a spectrum, the light would show up as energy at a fixed wavelength (not spread out over many wavelengths as in a continuous spectrum) and it is called an emission line. With the use of a Hydrogen Alpha filter, just this single frequency is recorded.

The Graph above shows the single frequency of light recorded with a Hydrogen Alpha filter shaded in blue. In this case it falls at a frequency of 656.3 nanometers.

The other green lines lower toward the blue end of the spectrum are Tripley Ionized Oxygen (OIII), and Hydrogen Beta (Hb). The red spectral lines represent light pollution frequencies from high pressure sodium and mercury vapor lights. Examining this graph, notice that all light pollution frequencies, OIII, and Hb are rejected and not passed through the Ha filter. To see an Emission Nebula recorded in Hydrogen Alpha CLICK HERE.