When an electromagnetic wave encounters a boundary between two different media, such as air and glass, part of the wave will be reflected back into the first medium, and part will be transmitted into the second medium. This phenomenon is known as reflection.

The reflection of electromagnetic waves can be understood using the laws of reflection and the properties of the waves. There are two primary laws of reflection:

- The angle of incidence (θi) is equal to the angle of reflection (θr). In other words, when an electromagnetic wave strikes a surface, it will be reflected off the surface at the same angle it approached.
- The incident wave, the normal to the surface, and the reflected wave all lie in the same plane.

When an electromagnetic wave is reflected, its electric and magnetic field components can undergo a phase change depending on the properties of the media at the boundary. This phase change is important for understanding the behavior of the reflected wave and any interference that may occur with other waves.

The reflection coefficient (R) represents the fraction of the incident power that is reflected at the boundary. It can be calculated using the Fresnel equations, which take into account the angles of incidence and reflection, as well as the properties of the two media (such as their refractive indices).

For normal incidence (θi = θr = 0), the reflection coefficient for the electric field (also called the amplitude reflection coefficient) can be calculated using the following formula:

R = |(n1 – n2) / (n1 + n2)|^2

where n1 and n2 are the refractive indices of the first and second media, respectively.

For non-normal incidence, the Fresnel equations become more complex and depend on the polarization of the incident wave. The incident wave can be decomposed into two orthogonal polarizations: transverse electric (TE) and transverse magnetic (TM). The Fresnel equations for TE and TM polarized waves will provide different reflection coefficients for each polarization.

Reflection of electromagnetic waves has many practical applications, such as in radar systems, communication systems, optics, and remote sensing.