normalincidence

Normal incidence refers to the orientation of a wave, such as light, with respect to a planar surface in which the beam travels perpendicular to the surface. The angle of incidence is measured relative to the surface normal, and at normal incidence this angle is 0 degrees.

In optics, normal incidence has several simplifying consequences. Because the incident ray is perpendicular to the surface, there is no refraction; the transmitted beam continues straight through the boundary according to Snell's law, although its amplitude can change due to reflection and transmission at the interface. The Fresnel equations simplify at normal incidence: the reflectance for power is R = ((n1 − n2)/(n1 + n2))^2 for lossless media, and the transmittance is approximately 1 − R when absorption is neglected. Polarization effects vanish at normal incidence, so the behavior is the same for different polarizations.

Normal incidence is a common simplifying assumption in optics and thin-film design, where coatings are often specified by their performance at this orientation. It is also a standard orientation in experimental measurements of reflectance, transmittance, or ellipsometry, where a beam is directed straight at a sample.

In practice, real surfaces are not perfectly flat; microscopic roughness or material anisotropy can cause diffuse reflection even at normal incidence. On curved or textured surfaces, the effective incidence angle can vary across the surface, reducing the meaning of a single normal incidence angle.

Typical applications include solar cells, optical detectors, and laser experiments, where aligning to normal incidence minimizes refraction and simplifies modeling.