Gas discharge lamps are a family of artificial light sources (or lamps). These lamps produce light by sending electricity through an ionized gas. Most of these lamps use a noble gas, or a combination of noble gases, but they often contain other materials, such as mercury, sodium or metal halides. The gases in these lamps are ionized in operation. This means that there are free electrons. When an electric current is sent through the gas, electrons collide with atoms of the gas and the metals. This will mean that some reach a higher energy state. When they fall back into the lower energy state, they will emit the energy in the form of light.