In 1913/1914, Franck and Hertz demonstrate that the collisions between electrons and gas molecules (in this case, neon) are elastic as long as the energy of the electrons lies below a certain threshold. When the threshold energy is attained, inelastic collisions occur and the neon radiates red light. According to quantum theory, the levels of energy transmitted are whole-number multiples of the threshold energy. This is why we observe discrete layers lighting up one after the other in a neon tube.