Physics
Franck-Hertz experiment
Electrons through mercury vapour: the collector current dips every 4.9 volts, because an atom takes that energy or none at all.
What happens
An electron gains energy along the tube and loses it in one lump the moment it can pay 4.9 eV to a mercury atom. That single rule makes the current collapse and recover as the accelerating voltage climbs. The glowing discs between the grids are where those inelastic collisions actually happened - in the real tube they are visible to the eye, and their number grows one at a time. The spacing in the readout is found by scanning the curve, not written into it.
Equations used in the module
Equations elastic below 4.9 eV; above it, inelastic: exactly 4.9 eV to the atom current dips every ΔV = 4.9 V E_Hg = 4.9 eV, λ_emitted = 254 nm
