The Fresnel term results from a complete analysis of the reflection process while considering light a an electromagnetic wave.

The electric field of light has an associated magnetic field associated with it (hence the name electromagnetic). The magnetic field is always orthogonal to the electric field and the direction of propagation. Over time the orientation of the electric field may rotate. If the electric field is oriented in a particular constant direction it is called polarized. The behavior of reflection depend on how the incoming electric field is oriented relative to the surface at the point where the field makes contact. This variation in reflectance is called the Fresnel effect.