image of the derivative of the optical path length in the direction of the difference in the transverse shift. In interferometry the illumination is split up into two beams by a partially reflective mirror. One of the beams is let through the sample before they are combined to interfere and give a direct image of the phase shifts. If the optical path length variations are more than a wavelength the image will contain fringes. There exist several phase-contrast X-ray imaging techniques to determine 2D or 3D spatial distribution of refractive index of samples in the X-ray regime. Applications The refractive index is an important property of the components of any optical instrument. It determines the focusing power of lenses, the dispersive power of prisms, the reflectivity of lens coatings, and the light-guiding nature of optical fiber. Since the refractive index is a fundamental physical property of a substance, it is often used to identify a particular substance, confirm its purity, or measure its concentration. The refractive index is used to measure solids, liquids, and gases. Most commonly it is used to measure the concentration of a solute in an aqueous solution. It can also be used as a useful tool to differentiate between different types of gemstone, due to the unique chatoyance each individual stone displays. A refractometer is the instrument used to measure the refractive index. For a solution of sugar, the refractive index can be used to determine the sugar content (see Brix). See also Fermat's principle Calculation of glass properties Clausius–Mossotti relation Ellipsometry Index-matching material Index ellipsoid Laser Schlieren Deflectometry Optical properties of water and ice Prism-coupling refractometry Phase-contrast X-ray imaging Velocity factor References External links NIST calculator for determining the refractive index of air Dielectric materials Science World Filmetrics' online database Free database of refractive index and absorption coefficient information RefractiveIndex.INFO Refractive index database featuring online plotting and parameterisation of data LUXPOP Thin film and bulk index of refraction and photonics calculations The Feynman Lectures on Physics Vol. II Ch. 32: Refractive Index of Dense Materials Dimensionless numbers Physical quantities Refraction