Why the bulb glows when the electric current passes through it?

Answer 1

the filament has electrical resistance. When electricity passes through it, it generates heat -- so much heat that the wire glows.

Answer 2

The filament in the bulb has resistance. The current flowing through the resistance of bulb causes the filament to dissipate energy in the form of heat and light. The filament is actually glowing white-hot because of the energy it is dissipating, thus giving off light. Any object having high temperature glows.