![]() The phenomenon of reflection arises from the diffraction of a plane wave on a flat boundary. When the light impinges perpendicularly to the surface, it is reflected straight back in the source direction. The law of reflection states that the angle of reflection of a ray equals the angle of incidence, and that the incident direction, the surface normal, and the reflected direction are coplanar. Reflection of the incident ray also occurs in the plane of incidence. When a ray encounters a surface, the angle that the wave normal makes with respect to the surface normal is called the angle of incidence and the plane defined by both directions is the plane of incidence. A ray of light is characterized by the direction normal to the wave front ( wave normal). Light propagates in space as a wave front of electromagnetic fields. The reflecting material of mirrors is usually aluminum or silver. A surface built from a non-absorbing powder, such as plaster, can be a nearly perfect diffuser, whereas polished metallic objects can specularly reflect light very efficiently. Matte paints exhibit essentially complete diffuse reflection, while glossy paints show a larger component of specular behavior. ![]() The distinction may be illustrated with surfaces coated with glossy paint and matte paint. Specular reflection reflects all light which arrives from a given direction at the same angle, whereas diffuse reflection reflects light in a broad range of directions. Reflection may occur as specular, or mirror-like, reflection and diffuse reflection. ![]() The Fresnel equations describe the physics at the optical boundary. In general, reflection increases with increasing angle of incidence, and with increasing absorptivity at the boundary. The degree of participation of each of these processes in the transmission is a function of the frequency, or wavelength, of the light, its polarization, and its angle of incidence. Optical processes, which comprise reflection and refraction, are expressed by the difference of the refractive index on both sides of the boundary, whereas reflectance and absorption are the real and imaginary parts of the response due to the electronic structure of the material. When light encounters a boundary of a material, it is affected by the optical and electronic response functions of the material to electromagnetic waves. ![]()
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