Although it resembles fluorescence, the light of a firefly is completely different. Firefly light is caused by the chemical reaction of a material. New molecules in a highly excited state are formed when the material is exposed to oxygen. Light will be emitted when returning to a normal low-energy state or ground state from an excited state. (Since no heat is produced, this is known as "cold light..")

The material known as luciferin in the bodies of fireflies oxidizes due to the enzyme luciferase. This is why fireflies glow when they breathe in and expand their abdomen. The spectrum of the greenish light fireflies produce is not a broad, continuous spectrum as with an electric lamp, but you certainly can make out the words in a book when they approach.