Index-matching material

In optics, an index-matching material is a substance, usually a liquid, cement (adhesive), or gel, which has an index of refraction that closely approximates that of another object (such as a lens, material, fiber-optic, etc.). When two substances with the same index are in contact, light passes from one to the other with neither reflection nor refraction. As such, they are used for various purposes in science, engineering, and art. For example, in a popular home experiment, a glass rod is made almost invisible by immersing it in an index-matched transparent fluid such as mineral spirits. Oil immersion In light microscopy, oil immersion is a technique used to increase the resolution of a microscope. This is achieved by immersing both the objective lens and the specimen in a transparent oil of high refractive index, thereby increasing the numerical aperture of the objective lens. Immersion oils are transparent oils that have specific optical and viscosity characteristics necessary for use in microscopy. Typical oils used have an index of refraction around 1.515. An oil immersion objective is an objective lens specially designed to be used in this way. The index of the oil is typically chosen to match the index of the microscope lens glass, and of the cover slip. For more details, see the main article, oil immersion. Some microscopes also use other index-matching materials besides oil; see water immersion objective and solid immersion lens. In fiber optics and telecommunications, an index-matching material may be used in conjunction with pairs of mated connectors or with mechanical splices to reduce signal reflected in the guided mode (known as return loss) (see Optical fiber connector). Without the use of an index-matching material, Fresnel reflections will occur at the smooth end faces of a fiber unless there is no fiber-air interface or other significant mismatch in refractive index. These reflections may be as high as −14 dB (i.e., 14 dB below the optical power of the incident signal).