Hydrogen-alpha

Hydrogen-alpha, also called H-alpha or Hα, a spectral line of the hydrogen atom. The emission line has a wavelength of 656.28 nm in air and 656.46 nm in a vacuum. This creates the color dark red. It is the first spectral line in the Balmer series. The spectral line is emitted when a hydrogen electron goes from the third electron shell to the second electron shell. H-alpha is used in astronomy. The Hydrogen-alpha emissions can be observed in emission nebulae and in the Sun's atmosphere (like the solar prominences and the chromosphere).

Balmer series
In the Bohr model of the atom, electrons are in "quantized" energy levels in the atom's nucleus. This means that electrons orbit the atom in different electron shells. These energy levels are written as the principal quantum number n = 1, 2, 3 ..., so the first energy level is called n = 1, the second energy level is called n = 2, etc. Electrons are only allowed to be in these shells. They cannot be between two states.

Electrons emit photons when the move to lower electron shell levels. When they move from n ≥ 3 to n = 2, they release photons that are in the Balmer series. Each part of the Balmer series is named. They are each given Greek letters: 
n = 3 to n = 2 is called Balmer-alpha or H-alpha, 
n = 4 to n = 2 is called Balmer-beta or H-beta, 
n = 5 to n = 2 is called Balmer-gamma or H-gamma, etc.

Moving from  n ≥ 2 to n = 1 gives off photons in the Lyman series: 
n = 2 to n = 1 is called Lyman-alpha, 
n = 3 to n = 1 is called Lyman-beta, etc.

H-alpha has a wavelength of 656.281 nm. This wavelength of light is red visible light in the electromagnetic spectrum. Astronomers can find ionized hydrogen in gas clouds by looking for H-alpha photons. 

800px|thumb|center|The four visible emission lines in the Balmer series. The red line at the right is H-alpha

References

Other websites
 Description of etalon filter by Colin Kaminski 
 MCE Membrane Filter

Category:Atomic physics
Category:Spectroscopy
Category:Hydrogen