shortly by thermionic emission the light bulb contains a

filament. this filament has a cathode and an anode. when we

switch on the bulb.then the cathode of the filament becomes

hot. due to this thermal electrons are ejected from the

cathode and move towards anode.on the path of electrons,

they make an elastic collision with the gas molecules

within the tube or filament. so the atoms of the gas are

raised to higher energy states. when these atoms dexcite

then light is emitted as predicted by Bohr theory