A mercury vapor lamp is a type of gas discharge lamp that produces light by passing an electric current through vaporized mercury. When the current flows, it excites the mercury atoms, causing them to emit ultraviolet light. This ultraviolet light then interacts with a phosphor coating on the inside of the lamp, producing visible light. These lamps are commonly used for outdoor lighting, such as in streetlights and parking lots, due to their bright, white light and energy efficiency. However, they can take a few minutes to reach full brightness and contain small amounts of mercury, which requires careful disposal.