What is a plasma?
Plasmas are partially or completely ionized gases and vapors, which besides ions and electrons also contain chemical radicals and a large number of electronically excited particles. Far more than 99 percent of the universe known to us is in a plasma state. A plasma can be e.g. ignited and maintained by an electromagnetic field.
Characteristic of each plasma is its luminescence which, depending on the type of gas and its pressure, shines violet, blue, green, yellow, orange, or reddish. The plasma luminescence is used in fluorescent tubes, e.g. as advertising signs. Argon tubes shine blue, neon tubes produce an orange-red light. However, vaporous liquids may also be used as the illuminating gas, for example in fluorescent lamps, which are generally filled with a mercury-argon mixture.