A fluorescent tube typically contains an inert gas (like argon) and a small amount of mercury vapour at low pressure. When an electric voltage is applied across the electrodes, it accelerates electrons within the tube. These electrons collide with the gas atoms and mercury atoms, causing them to become ionized (losing electrons) and excited. This creates a state of matter consisting of a collection of positively charged ions and negatively charged electrons, along with neutral atoms, that is overall electrically neutral. This state is known as plasma. The excited mercury atoms and ions emit ultraviolet (UV) light. The inside of the tube is coated with a phosphorescent material (phosphor) that absorbs the UV light and re-emits it as visible light, causing the tube to glow.