Question

why incomplete combustion is luminous?

Answer 1

in incomplete combustion, the carbon atoms which are in layman's language half burned or left un-burnt absorbs the heat energy we provided. this heat energy excites the electrons of the carbon atom and they go to higher orbitals and while coming back to their original state they emit the energy absorbed in form of light energy.
in simple words, when the half burned carbons absorb energy in form of heat they release it in form of light. thus incomplete combustion is luminous.