Question

there is a vacuum between two glass tubes. What is the importance of the vacuum ????

Answer 1

In electronics, a vacuum tube, an electron tube,[1][2][3] or just a tube (North America), or valve (Britain and some other regions)[4] is a device that controls electric current between electrodes in an evacuated container. Vacuum tubes mostly rely on thermionic emission of electrons from a hot filament or a heated cathode. This type is called a thermionic tube or thermionic valve. A phototube, however, achieves electron emission through the photoelectric effect. Not all electronic circuit valves/electron tubes are vacuum tubes (evacuated); gas-filled tubes are similar devices containing a gas, typically at low pressure, which exploit phenomena related to electric discharge in gases, usually without a heater

Answer 2

If the aforesaid points towards the demand of a cathode ray discharge tube setting then perhaps the presence and the significance of the vacuum can be explained. The fluorescence or phosphorescence that will be visible once the negative beam strikes the surface would be exposed if and only if there are no externally draining forces like the minute magnetic fluxes and voltage affected gases inducing an oppositely charged beam instead . That is why the tube is evacuated.