Final answer:
Spectral lines appear as vertical lines because they represent the wavelengths of light corresponding to specific energy transitions in atoms, as understood through Bohr's model. The Balmer series and the Zeeman effect show examples of how energy levels in atoms define these wavelengths.
Step-by-step explanation:
Spectral lines in an emission or absorption spectrum appear as distinct vertical lines because they denote wavelengths of light that correspond to specific energy transitions within an atom. According to Bohr's model, each spectral line represents a transition between energy levels in an atom, either absorbing or emitting photons of precise energies or wavelengths. Electrons can move from one energy level to another, and the emitted or absorbed light's wavelength indicates the energy change associated with this transition.
For instance, the emission lines observed in the hydrogen atom correspond to series like the Balmer series, with transitions ending at the n=2 energy level and producing visible light. In the presence of a magnetic field, the Zeeman effect causes each energy level to split into multiple, closely spaced levels, resulting in a series of spectral lines.
Overall, the phenomenon of discrete spectral lines is deeply connected to the atomic structure and the specific energies at which electrons orbit the nucleus. These lines are not arbitrary but are precisely determined by the physical properties of the atoms involved.