The hydrogen emission spectrum is a fascinating display of light that is emitted when hydrogen gas is energized. However, when we observe the spectrum, we often see only four distinct lines, known as the Balmer series. These lines correspond to specific wavelengths of light that are emitted as electrons in hydrogen atoms transition between energy levels.
The primary reason we see only four lines is due to the limitations in the energy transitions that occur within a hydrogen atom. Hydrogen has one electron, and when this electron is excited to a higher energy level, it can fall back down to one of the lower levels, releasing energy in the form of light. In the case of the Balmer series, these transitions specifically involve the second energy level.
The four lines correspond to the following transitions: from the third level to the second (H-alpha), from the fourth level to the second (H-beta), from the fifth level to the second (H-gamma), and from the sixth level to the second (H-delta). Each of these transitions emits light at a unique wavelength, which is why we see distinct lines in the spectrum.
It’s worth noting that hydrogen can emit light in other series (such as the Lyman and Paschen series), but those wavelengths fall outside the visible spectrum or correspond to other energy levels. Thus, for visible light, the four lines from the Balmer series are the only ones typically observed in the hydrogen spectrum.