When a (laser) light beam is reflected at the base of a (glass) prism, the light is reflected according to Snell’s law at the interface. Above a critical angle > total internal reflection (TIR) occurs (Figure).
The critical angle is determined by the refractive indices of the prism, and the material that is in contact with the base side of the prism, respectively.
In many cases the latter will be a liquid phase that contains the analyte, if a (bio)sensing event is considered.
In the event of total internal reflection, the light is not completely reflected at the base side of the prism, but a rather small amount of light passes the interfaces and propagates parallel to the plane of reflection.
These “evanescent waves” generate what is known as the evanescent field (Figure). The evanescent field shows an exponential decrease in the normal direction of the plane of reflection.