Light has a dual nature: it is electromagnetic waves as well as trains of photons. A photon is a packet of energy (quantum) and exhibits properties of both a wave and a particle. When a photon of adequate energy hits a molecule, it is absorbed and brings the molecule to, what is called, an excited state. Thanks to this extra amount of energy, the molecule acquires different properties compared to its previous normal state. Nevertheless, it does not reside for long in this excited state and tends to get rid of its extra energy taking one of the following three routes: i) it may release the extra energy as heat to its surrounding, ii) it may undergo a chemical reaction leading to a new product, iii) it may emit a photon as fluorescence or phosphorescence.