In optical fiber installation, referring to light reflected back toward the source from the air gap at the point where two fiber endfaces meet in a connector. There is always a slight air gap, as the endfaces are never perfectly cleaved and never can be aligned in the connectors so that they mate perfectly. The considerable difference in index of refraction (IOR) between glass and air causes some amount of light to reflect. An angled physical contact (APC) connector, which joins two fiber endfaces at a slight angle, sometimes is used to minimize attenuation and back reflection. See also APC, attenuation, backscatter, connector, IOR, and optical fiber.