Franck-Hertz Apparatus 

[Neon Gas]

The Franck-Hertz experiment with neon gas was a variation of the original experiment using mercury, and aimed to show that neon atoms also absorb a certain (quantized) amount of energy. In this experiment, electrons are accelerated through a tube filled with neon gas. When the energy of the electrons reaches a certain value (around 18.7 eV), the electrons will inelastically collide with the electrons of the neon atoms, causing the excitation of the electrons of the neon atoms. This excitation produces a characteristic glowing light (usually reddish-orange) that can be observed inside the tube. Like the mercury version, this experiment provided strong evidence that the energy in atoms is discretized, supporting quantum theory.