The refractive index plays a crucial role in Fresnel’s equations because it determines how light behaves when it crosses the boundary between two different media. It directly affects how much of the light is reflected and how much is transmitted.
Each material has a refractive index, which measures how much it slows down light compared to a vacuum. When light passes from one medium to another—like from air into water—the difference in their refractive indices causes the light to bend (refract) and also affects the strength of the reflected and transmitted waves.
In Fresnel’s equations, the refractive indices of both the first and second media are key factors in calculating the reflection and transmission coefficients. A larger difference between the two refractive indices usually results in more reflection at the boundary. For example, light reflecting off glass from air reflects more strongly than from water.
The refractive index also determines the critical angle for total internal reflection and the Brewster angle where p-polarized light is not reflected at all. In essence, the refractive index defines how light interacts with the boundary—how much is reflected, how much is transmitted, and how it changes direction—making it fundamental to the predictions made by Fresnel’s equations.