usually has a real part smaller than 1. It is therefore normally written as (or with the alternative convention mentioned above). Far above the atomic resonance frequency delta can be given by where is the classical electron radius, is the X-ray wavelength, and is the electron density. One may assume the electron density is simply the number of electrons per atom Z multiplied by the atomic density, but more accurate calculation of the refractive index requires replacing Z with the complex atomic form factor . It follows that with and typically of the order of 10−5 and 10−6. Relations to other quantities Optical path length Optical path length (OPL) is the product of the geometric length d of the path light follows through a system, and the index of refraction of the medium through which it propagates, This is an important concept in optics because it determines the phase of the light and governs interference and diffraction of light as it propagates. According to Fermat's principle, light rays can be characterized as those curves that optimize the optical path length. Refraction When light moves from one medium to another, it changes direction, i.e. it is refracted. If it moves from a medium with refractive index n1 to one with refractive index n2, with an incidence angle to the surface normal of θ1, the refraction angle θ2 can be calculated from Snell's law: When light enters a material with higher refractive index, the angle of refraction will be smaller than the angle of incidence and the light will be refracted towards the normal of the surface. The higher the refractive index, the closer to the normal direction the light will travel. When passing into a medium with lower refractive index, the light will instead be refracted away from the normal, towards the surface. Total internal reflection If there is no angle θ2 fulfilling Snell's law, i.e., the light cannot be transmitted and will instead undergo total internal reflection. This occurs only when going to a less optically dense material, i.e., one with lower refractive index. To get total internal reflection the angles of incidence θ1 must be larger than the critical angle Reflectivity Apart from the transmitted light there is also a reflected part. The reflection angle is equal to the incidence angle, and the amount of light that is reflected is determined by the reflectivity of the surface. The reflectivity