Question

Why does the hydrogen spectrum consist of lines?

Answer 1

Though a hydrogen atom has only one electron, it contains a large number of shells, so when this single electron jumps from one shell to another, a photon is emitted, and the energy difference of the shells causes different wavelengths to be released... hence, mono-electronic hydrogen has many spectral lines

Answer 2

As the electrons fall from higher levels to lower levels, they release photons. Different "falls" create different colors of light. A larger transition releases higher energy (short wavelength) light, while smaller transitions release lower energies (longer wavelength).

The visible wavelengths are caused a by single electron making the different transitions shown below. There are even more transitions that release invisible wavelengths.