1. The problem statement, all variables and given/known data What is the yellowish light beam in the Teltron tube before the accelerating voltage is raised up? 2. Relevant equations - 3. The attempt at a solution When we switch on the device, the initial light beam is in yellow color. When we adjust the accelerating voltage to a certain degree, a bluish light beam appear. We think that the reason for the presence of light beam in different color (from yellowish to bluish) is the difference in the energy excitation level of the helium gas due to the bombardment of the electrons with the helium gas particle. So, since the electrons are initially at ground state, the light beam fluoresces in yellowish color. Does the above explanation valid for the presence of yellowish light beam during the beginning of the experiment. Hope someone can enlighten me. Thank you in advance.