Why does gas discharge emit light?

In everyday life, the fluorescent lights and neon signs we see often rely on gas discharges to generate light. So, what's the difference between gas discharges and current conduction in metal conductors?

1. Current in Solid Conductors

In metal conductors, current is essentially the movement of free electrons within a fixed ion lattice.

The atoms and ions themselves are mostly stationary, undergoing only thermal vibrations.

The number of free electrons is roughly equal to the number of ions, making metals highly conductive.

The direct result of current flowing through them is heat. Only when the temperature reaches incandescence does light (such as the red or white glow of a filament) appear.

2. Current in Gases

Gases are not conductive under normal circumstances because they contain almost no free electrons. Only when the atoms in the gas are ionized, producing electrons and ions, do they become conductive.

In a discharge tube:

Electrons move toward the anode at high speed and constitute the majority of the current.

Positive ions move toward the cathode, but due to their large mass and slow speed, their contribution to the current is typically only 0.1%–1%.

Therefore, the total current in a gas discharge is essentially primarily electron current.

3. Why does a gas discharge emit light?

The key lies in collisional excitation.

After being accelerated by an electric field, high-speed electrons collide with gas atoms or molecules, exciting them to higher energy levels.

When these excited atoms return to their ground state, they release photons.

This type of luminescence is not due to incandescent temperatures, but rather to energy level transitions of atoms or molecules.

This is why different gases emit different colors of light; for example, neon lamps emit red-orange light, while argon lamps emit blue-violet light.

To summarize:

Metal conductors emit light through heating, while gas discharges emit light through electron-excited atoms—two completely different physical mechanisms.