Neon is neither shiny nor dull, as its light comes from within rather than being reflected or absorbed. “Shiny” describes a surface that reflects light, while “dull” refers to one that absorbs light. Neon is a gas that becomes luminous when energized. When electricity is applied, neon atoms emit their own light, a process called luminescence. This light emission transforms electrical energy into the visible glow seen in signs.
Neon: An Inert Gas with Unique Properties
Neon is a member of the Group 18 elements on the periodic table, known as the noble gases, and it has an atomic number of 10. The structure of the neon atom features a stable arrangement of ten electrons, with its outermost energy shell completely filled. This full outer shell makes neon chemically unreactive, classifying it as an inert gas that does not readily form compounds.
This chemical stability is why neon is suitable for use in gas-discharge tubes for lighting. Since the gas does not react with the metal electrodes or the glass walls of the tube, it ensures the long life and consistent performance of the sign. The gas is sealed inside the evacuated glass tube at a very low pressure, creating the necessary environment for the electrical discharge.
The Physics of the Red-Orange Glow
The iconic glow of a pure neon sign begins with the application of high voltage electricity across the electrodes at opposite ends of the sealed glass tube. This potential difference creates a powerful electric field inside the tube. The energy from the field accelerates the few free electrons already present within the low-pressure neon gas.
As these accelerated electrons travel through the tube, they collide with the neutral neon atoms. The force of these collisions strips electrons from the neon atoms (ionization), creating positively charged ions and a flood of new free electrons. This electrically charged gas mixture is a plasma, often referred to as the fourth state of matter.
The continuous flow of electrons and ions sustains the plasma. Electrons constantly collide with and transfer energy to other neon atoms. This energy transfer forces the electrons of the neon atoms to jump from their stable, lower energy levels to temporary, higher energy levels, a phenomenon known as excitation.
The excited state is highly unstable. When the electron drops back down to its original, lower energy level, the excess energy absorbed is released as a packet of light energy, called a photon. Because the energy levels of a neon atom are fixed and distinct, the amount of energy released corresponds to a specific wavelength of light, which is why pure neon gas always emits its signature red-orange color.
Creating the Rainbow: Why “Neon” Signs Use Other Elements
The common perception that all brightly colored signs are filled with pure neon gas is a misconception, as neon only produces the red-orange light. To achieve the full spectrum of colors seen in commercial signs, manufacturers must employ other noble gases or a combination of gases and special coatings. For instance, argon gas is frequently used for cooler colors and gives off a blue or lavender light when energized.
Argon is often mixed with a tiny amount of mercury vapor to create a more vibrant light output. When the argon-mercury mixture is excited by electricity, the mercury atoms emit a significant amount of ultraviolet (UV) light, which is invisible to the human eye. This UV light is then harnessed to create a wide range of colors.
Sign makers coat the inside of the glass tubes with various phosphor powders, which are fluorescent materials. When the invisible UV light from the excited gas mixture strikes this phosphor coating, the coating absorbs the UV energy and re-emits it as visible light in a specific color. Different chemical compositions of the phosphor coating allow for the creation of yellows, greens, pinks, and whites. This technique allows for a broad palette of colors far beyond the natural red-orange glow of pure neon.