The forerunner for the iconic neon light was first demonstrated in 1855 by the German physicist Heinrich Geissler. He observed that a soft glow was emitted when an electric field was applied across a gas tube containing a low-pressure gas. We now understand that the applied electric field was stripping electrons from the atoms in the gas, and creating a flow of negative electrons in one direction and positively charged, ionized atoms (ions) in the other. Collisions between fast-flowing ions or electrons with atoms would lead to further ionizations, thus perpetuating the process. Such a mixture of electrons and ions is now referred to as a plasma. While we normally think of solids, liquids, and gases as the only three states of matter, physicists count the plasma as a fourth.
DEFINITION
A plasma is a state of matter formed when an atomic or molecular gas is highly ionized, such that it is almost entirely composed of ionized atoms or molecules and free electrons.
The collisions between ions or electrons and atoms don’t always have enough energy to liberate atomic electrons. When they don’t, the colliding atoms are merely boosted from their ground state into some excited state. A short time later, the atom will make the downward transition to a lower lying state, thus emitting a photon at a characteristic frequency set by the spacing between energy levels. These photons are responsible for the glow, and the characteristic frequency determines its color. Red light is emitted when a glow discharge uses neon, while helium emits a purple color, carbon dioxide emits a white color, and mercury emits a blue color. This is therefore a straightforward example of the atomic transitions we discussed.
This fundamental physical process inspired the French engineer Georges Claude to file a patent for the technology in France in 1910, which led to the use of neon lights for advertising and art. The glow discharge is also the basis for the sodium-vapor lamp, whose soft yellow-orange glow is used to light countless streets around the world.
A miniaturized device operating on the same principles, the neon glow lamp, was introduced in 1917. These were widely used through the 1970s for electronics displays, and today serve as the basic technology for plasma televisions and displays.
Even ordinary, everyday fluorescent light bulbs are based on the glow discharge. Here, the discharge emissions come from mercury vapor, and these are actually in the (invisible) ultraviolet range. The ultraviolet photons are then used to excite phosphorescent wall coatings, which in turn emit the bright white light so common in our daily lives.
Neon lights, street lamps, and fluorescent lighting are all based on the glow discharge, in which a particular frequency of light is emitted through a carefully selected atomic transition. Characteristic wavelengths for each are shown in these simplified energy level diagrams.
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