Fresnel’s equations apply to the interface between two different media—such as air and glass, or water and plastic—by describing how light is split into reflected and transmitted parts when it hits the boundary. These equations take into account the refractive indices of both media, the angle of incidence, and the polarization of the incoming light.
When a light wave reaches the boundary between two media, part of it is reflected back into the original medium, and part of it is transmitted (or refracted) into the second medium. Fresnel’s equations give precise values for the fraction of light that is reflected and transmitted. The behavior depends on whether the light is s-polarized (electric field perpendicular to the plane of incidence) or p-polarized (electric field parallel to the plane of incidence), because these orientations interact differently with the boundary.
If the second medium is optically denser, more light tends to be reflected at higher angles. If the light travels from a denser to a rarer medium and the angle of incidence exceeds a certain limit (the critical angle), total internal reflection occurs, and no light is transmitted.
Fresnel’s equations are fundamental in understanding light behavior at surfaces and are widely used in optics, including lens design, anti-reflective coatings, fiber optics, and imaging systems.