I learned from a website that cathode ray tubes fire electrons from an "electron gun" at the cathode towards the anode, hitting a piece of metal and produce cathode rays in the process. My question is; how do cathode tubes "shoot" electrons? Apparently it's due to a high potential difference between the anode and cathode, but wouldn't that create an arc of electricity instead? Also, why do the rays turn from blue to green when the electrons hit the piece of metal? I'm curious.