The light emitted by firefly is due to

The light emitted by a firefly is a result of bioluminescence, which is a specific type of chemiluminescence [t1][t6]. This process involves a chemical reaction where energy is released in the form of light rather than heat, often referred to as 'cold light' [t3][t4]. In fireflies, this reaction occurs when the enzyme luciferase acts upon the substrate luciferin in the presence of oxygen and adenosine triphosphate (ATP) [t3][t8]. The reaction excites the luciferin molecules to a higher energy state; as they return to their ground state, they emit photons of visible light [t5][t8]. Unlike combustion or the burning of phosphorus, which are exothermic processes releasing significant heat [c2][c5], firefly bioluminescence is highly efficient, ensuring the insect's survival by avoiding thermal damage [t3]. It is not related to radioactivity [c4] or photoelectric effects.

Sources

1. [1] https://education.nationalgeographic.org/resource/bioluminescence/
2. [2] https://www.scientificamerican.com/article/how-and-why-do-fireflies/
3. [3] Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > 5.3.2 Combustion > p. 62
4. [4] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > 4.3.1 Combustion > p. 69