The discharge tube contains a low pressure of a rare gas such as argon and mercury vapor. The mercury atoms are provided by a drop of liquid mercury, or by a semi-solid amalgam of mercury and other metals such as bismuth, lead, or tin. Some of the liquid mercury or the mercury in the amalgam vaporizes to provide the mercury vapor. The electric field ionizes some of the mercury atoms to produce free electrons, and then accelerates those free electrons. When the free electrons collide with mercury atoms, some of those atoms absorb energy from the electrons and are "excited" to higher energy levels. After a short delay, the excited mercury atoms spontaneously relax to their original lower energy state and emit a UV photon with the excess energy. As in a conventional fluorescent tube, the UV photon diffuses through the gas to the inside of the outer bulb, and is absorbed by the phosphor coating that surface, transferring its energy to the phosphor. When the phosphor then relaxes to its original, lower energy state, it emits visible light. In this way the UV photon is down-converted to visible light by the phosphor coating on the inside of the tube. The glass walls of the lamp prevent the emission of the UV photons because ordinary glass blocks UV radiation at the 253.7 nm and shorter wavelengths.