How does the pressure inside a mercury vapor lamp affect its spectral distribution, particularly as a source of ultraviolet light?
At low pressure, a mercury vapor lamp emits mostly short wavelength ultraviolet light at a wavelength of 254 nanometers. This light comes from the dominant atomic transition in the mercury atom, between its first excited state and its ground state. However, as the pressure and density of mercury atoms inside the lamp increase, two things happen. First, the high density of mercury atoms in the lamp makes it difficult for the 254-nanometer light to escape from the lamp. Each time a 254-nanometer photon (particle of light) is emitted by one mercury atom, a nearby mercury atom absorbs it. As a result, the 254-nanometer light becomes trapped inside the lamp and diminishes in brightness. With so much energy trapped inside the lamp, the mercury atoms are able to reach more highly excited states than at low density. Second, frequent collisions between the now highly excited mercury atoms allow those mercury atoms to emit wavelengths of light that are normally forbidden in the absence of collisions. The mercury atoms begin to emit light at a wide variety of wavelengths, including substantial amounts of visible light. That’s why a high-pressure mercury lamp is a brilliant source of visible light—most of the ultraviolet light is trapped by the mercury vapor and a substantial fraction of the light emerging from the lamp is visible light.