A plasma tube is usually filled with a low-pressure noble gas such as neon, argon, or a gas mixture. Electrodes at the ends of the tube apply a high voltage across the gas. Under normal conditions, gases are electrical insulators, but when the voltage becomes high enough, electrons are pulled away from atoms in the gas. This process is called ionization. Once ionization begins, the gas becomes a plasma: a mixture of free electrons, positively charged ions, and neutral atoms. Because these particles carry charge, the plasma can conduct electricity. The electric field inside the tube accelerates the free electrons, causing them to collide with other atoms in the gas. These collisions do two important things: 1. They knock additional electrons free, sustaining the plasma. 2. They excite atoms, pushing their electrons into higher energy states. When those excited electrons fall back down to lower energy levels, they release energy in the form of photons—packets of light. The specific ...