This interactive simulation allows users to understand atomic excitations due to electron scattering. The user can set the voltage of the electrodes in a simulated discharge tube and  view the energy level transitions and spectra of photons emitted from atoms excited by the electrons in the tube. Either hydrogen atoms or atoms with energy levels defined by the user can be studied. The number of energy levels in the configurable atoms, the electron current, and whether one or many atoms are simulated can be set by the user. Processes can be viewed in slow motion or step by step.

This simulation is part of a large and growing collection. It has been designed using principles from physics education research and refined based on student interviews.