Editing Franck–Hertz experiment (section)

== Experiment with neon ==
In instructional laboratories, the Franck–Hertz experiment is often done using [[neon]] gas, which shows the onset of inelastic collisions with a visible orange glow in the vacuum tube, and which is non-toxic, should the tube be broken. With mercury tubes, the model for elastic and inelastic collisions predicts that there should be narrow bands between the anode and the grid where the mercury emits light, but the light is ultraviolet and invisible. With neon, the Franck–Hertz voltage interval is 18.7 volts, and an orange glow appears near the grid when 18.7 volts is applied.  This glow will move closer to the cathode with increasing accelerating potential, and indicates the locations where electrons have acquired the 18.7&nbsp;eV required to excite a neon atom. At 37.4 volts two distinct glows will be visible: one midway between the cathode and grid, and one right at the accelerating grid. Higher potentials, spaced at 18.7 volt intervals, will result in additional glowing regions in the tube.

An additional advantage of neon for instructional laboratories is that the tube can be used at room temperature. However, the wavelength of the visible emission is much longer than predicted by the Bohr relation and the 18.7&nbsp;V interval. A partial explanation for the orange light involves two atomic levels lying 16.6&nbsp;eV and 18.7&nbsp;eV above the lowest level. Electrons excited to the 18.7&nbsp;eV level fall to the 16.6&nbsp;eV level, with orange light emission.<ref>{{cite book |title=Fundamentals of Light Sources and Lasers |pages=31–36 |chapter=2.6 The Franck–Hertz Experiment |first=Mark |last=Csele |publisher=John Wiley & Sons |year=2011 |isbn=9780471675228 |chapter-url=https://books.google.com/books?id=xQfKWwvH42kC&pg=PA31}}</ref>

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File:This file shows the Franck-Hertz experiment with Neon resulting in glowing regions appearing.webm|Franck-Hertz experiment with neon resulting in glowing regions appearing
File:Franck–Hertz Experiment.png|Franck-Hertz experiment with neon gas: three glowing regions
File:Franck-Hertz experiment with neon.png|Franck-Hertz experiment with neon: Anode current versus grid voltage (relative to the cathode).
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