Reflexionsphase

Reflexionsphase refers to the phase shift that a wave acquires when it is reflected at the boundary between two different media. It is a key component of the Fresnel equations and plays a central role in optics, acoustics and quantum mechanics. The amount of phase change depends on the properties of the two media, the angle of incidence, and, for light, the polarization of the wave.

In many ordinary cases, the reflected wave has either a phase shift of 0 or of 180 degrees (pi). For normal incidence in optics, the amplitude reflection coefficient is r = (n1 − n2)/(n1 + n2), where n1 and n2 are the refractive indices. If n2 > n1 (the wave goes from lower to higher refractive index), r is negative and the reflected wave undergoes a pi phase shift. If n2 < n1, r is positive and there is no phase reversal. At oblique incidence, the phase shifts for s- and p-polarized light can differ, and the Fresnel equations provide the respective phase changes.

In acoustics, the phase change upon reflection depends on the acoustic impedances of the media. The sign and magnitude of the reflection coefficient determine whether the reflected wave experiences a phase inversion, a partial phase shift, or none, again influenced by incidence angle and polarization (where applicable).

Reflexionsphase has practical consequences in interference phenomena such as thin-film coatings, dielectric mirrors and anti-reflective layers, where the cumulative phase shifts at multiple interfaces determine constructive or destructive interference. It also features in quantum mechanical scattering, where reflected waves acquire energy- and angle-dependent phase shifts that affect interference patterns.