Ultraviolet Lamp radiation of 254 nm wavelength must hit the microorganism to inactivate it, and each microorganism must absorb a specific amount of energy (dosage) to be destroyed.
Proteins and nucleic acid, which all microorganisms contain as their main constituents, absorb UV radiation energy. After absorption, the UV energy destroys or inactivates the DNA (deoxyribonucleic acid), thus preventing the microorganisms from reproducing.