Final answer:
The hydrogen vapor lamp must be turned on before it emits light because the electrons in hydrogen atoms need to be excited to higher energy levels. Once these electrons return to lower levels, they emit light of specific colors.
Step-by-step explanation:
The hydrogen vapor lamp must be turned on before it gives off light due to the process of photon activation. When the lamp is turned on, energy is provided that heats the hydrogen gas inside. As the gas heats up, the electrons within the hydrogen atoms are excited to higher energy levels. When these excited electrons transition back to lower energy states, they emit light of specific wavelengths or colors. This is a typical example of atomic excitation and de-excitation which explains observable phenomena such as the emission spectra of gases when they are heated.