Final answer:
Spectral lines split in the presence of external magnetic or electric fields due to the fields' interactions with the electrons' magnetic moments (Zeeman effect) or electric dipole moments (Stark effect), revealing the existence of atomic subshells.
Step-by-step explanation:
The reason that the spectral lines of an atom split when an external magnetic or electric field is applied is due to the interaction of these fields with the magnetic moments of electrons, which are associated with their orbital and spin angular momenta. This interaction results in a phenomenon known as the Zeeman effect, where the energy levels of electrons within an atom become split into multiple, closely spaced levels.
Each of these new levels corresponds to a different orientation of the electron's magnetic moment in the external field, leading to additional allowed transitions and thus, a splitting of the spectral lines. The Stark effect, similarly, results from the interaction of an external electric field with the electric dipole moment of the atom, causing further splitting. These effects are crucial in that they provide evidence for the existence of subshells within an atom, based on the patterns of splitting observed which correlate to different subshell configurations.
For example, if no external magnetic field is present, transitions from a particular orbital might result in a single spectral line. However, once a magnetic field is applied, this line can be split into three, five, or more lines, depending on the initial state's azimuthal quantum number (l), which is directly related to the subshell. The splitting pattern gives insight into the fine structure of the energy levels in an atom, which can only be explained by the presence of subshells and finer details such as electron spin and magnetic interactions within the atom.