The Open Door Web Site  
Atomic and Nuclear Physics Spectra and Energy Levels in Atoms Balmer measured the wavelengths of visible light emitted by hydrogen atoms. He found that they could be described by the following formula (now called the Balmer series).
is wavelength, R is a constant (the Rydberg constant) and n = 3, 4, 5, etc. In order to explain the Balmer series, Bohr made the following suggestions
Energy transitions in atoms are often represented by diagrams like the one below. The lowest energy is called the "ground state".

