Combustion is the process of burning of a substance in the presence of oxygen. The fuel (coal or petroleum) gets oxidized. As a result, the bonds present in the molecules of fuel break, releasing energy. Sometimes, the energy released falls in the visible range of the electromagnetic spectrum (which means the energy released is visible to human eye) which we call as light. Sometimes the energy released falls under infrared region, which we commonly call as heat. Basically, the compounds generate heat or light or both due to the release of energy from the breaking of molecular bonds.