What is the hydrogen spectrum?

The hydrogen spectrum is a series of spectral lines with specific wavelengths emitted by atomic hydrogen as a result of electron transitions between two energy levels in the atom.
The Bohr’s model of the hydrogen atom explains the spectrum of hydrogen. Bohr’s explanation is that when an electron from one orbit transitions into another orbit of lower energy, it emits energy equal to the difference between the energy of the two orbits in the form of small packets of light known as photons. Each transition of an electron gives off a line in the spectrum.
The emission spectrum of hydrogen is best explained by describing what happens when electric current passes through hydrogen gas in a glass tube at a low pressure. It gives off a blue light which splits into four narrow bands of bright light of specific wavelengths and colours when passed through a prism.
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch6/bohr.html