As they heat up, the rising carbon atoms (as well as atoms of other material) emit light. This "heat produces light" effect is called incandescence, and it is the same kind of thing that creates light in a light bulb. It is what causes the visible flame.

Light is emitted from flames by two primary mechanisms: one is small particles glowing incandescently because they are hot (the same mechanism that drives an incandescent light bulb); the other is from electronic transitions from specific energy levels in excited atoms in the flame produces as a by product of the combustion process (this is partly why flames can coloured specifically by elements present in the burning material).

The process, called pyrolysis, releases atoms and energy. Unbound atoms form a hot gas, mingling with oxygen atoms in the air. This glowing gas — and not the fuel itself — produces the spooky blue light that appears at the base of a flame.