1. The problem statement, all variables and given/known data From the book: "When electrodes are placed in a gas at normal atmospheric pressure no current passes and the gas act as an insulator until the electric field is increased to above 3 or 4 MV/m when sparking takes place. In crontast, at lower pressures, a steady current can be mantained in a gas. At pressures of about 1mm of mercury, the discharge is accompanied by the enmision of light, but at still lower pressures a dark region forms near the cathode. The dark region, called the Crookes dark space, increases in size as the pressure falls, filling the discharge tube at pressures of 0.001 mm and below. If under these low-pressure conditions, a small hole is made in the anode, a green glow is observed on the glass wall of the discharge tube." 3. The attempt at a solution I'm not sure about mine explanation of the phenomena: if you have a great ddp, you can see light from cathode to anode becouse the electrons hits gas particle and ionize them. When the ddp falls the electron has not enough energy.. so you can only see a ray outside of the tube because they want to loose their momentum by releasing photons? sorry form my bad english and thanks all Mahblah.