Surface plasmon resonance (SPR) is an optical technique that measures the refractive index changes at a metal-dielectric interface, primarily used to detect molecular interactions in real-time. By shining light onto a thin metal film, such as gold or silver, SPR detects the resonant oscillation of surface electrons, known as plasmons, which occurs when the frequency of incident light matches the natural frequency of these electrons. This method has become vital in biosensing applications, allowing for the study of biomolecular interactions without the need for labels.