Final answer:
Spectral lines are most distinct under low pressure because at high pressure the lines become broader due to increased collisions among the gas particles.
Step-by-step explanation:
The spectral lines in a mercury vapor lamp are most distinct when viewed under low pressure. At higher pressures, the emission lines become broader due to more frequent collisions between atoms or molecules, which can cause what is known as pressure broadening. Observations of spectral behavior in different gases show that elements emit specific wavelengths of light when excited, and these are best observed when the gas is at low pressure. For example, when an electrical discharge passes through hydrogen gas at low pressure, we see a line spectrum indicating discrete photonic energies, rather than a continuous spectrum.