Final answer:
A hydrogen vapor lamp needs to be turned on before it gives off light because the gas inside the lamp needs to be energized first. The energization process causes the electrons within the hydrogen atoms to transition to higher energy levels, and as they transition back to lower energy levels, they emit light of specific colors.
Step-by-step explanation:
When a hydrogen vapor lamp is turned on, the gas inside the lamp is energized. This energization causes the electrons within the hydrogen atoms to transition to higher energy levels. As these electrons then transition back to lower energy levels, they emit light of specific colors. Therefore, the lamp must be turned on before it gives off light because it needs this initial energization process to produce the emission of light.