At an interface between two transparent media of different refractive index (glass and water), light coming from the side of higher refractive index is partly reflected and partly refracted. Above a certain critical angle of incidence, no light is refracted across the interface, and total internal reflection is observed. While incident light is totally reflected the electromagnetic field component penetrates a short (tens of nanometers) distance into a medium of a lower refractive index creating an exponentially detenuating evanescent wave. If the interface between the media is coated with a thin layer of metal (gold), and light is monochromatic and p-polarized, the intensity of the reflected light is reduced at a specific incident angle producing a sharp shadow called surface plasmon resonance due to the resonance energy transfer between evanescent wave and surface plasmons

Surface plasmon resonance is a theory which is easy to understand. SPR is being widely used in the manufacturing of various sensors. It is used for gas sensing, chemical sensing more importantly in biosensing. It is also used in microscopic imaging.