A mercury-vapor lamp is a type of gas discharge lamp that produces light through the excitation of mercury vapor. It consists of a glass bulb filled with low-pressure mercury vapor and an inert gas, such as argon. When an electrical current passes through the bulb, it ionizes the gases, resulting in a discharge that emits ultraviolet (UV) light. This UV light then excites the mercury vapor, causing it to emit visible light primarily in the blue and green spectrum.