In nonlinear optics, the harmonics of light refer to optical frequency conversion phenomena that occur when intense laser light interacts with nonlinear optical materials such as specific crystals or gases. Through processes like second-harmonic generation and third-harmonic generation, incoming photons combine within the crystal lattice to generate new coherent light photons vibrating at exact integer multiples of the fundamental laser frequency. For instance, passing an infrared laser beam through a specialized nonlinear crystal can double its frequency to produce visible green light or triple it into the ultraviolet spectrum. This advanced optical technology is widely utilized in laser physics, telecommunications, medical equipment, and scientific research to expand available laser wavelengths.