Refractive index

Refractive index is the ratio of light’s speed in vacuum to its phase velocity in a material. It determines how light’s phase and direction change when light enters that material.

Snell's law: A law relating the angles of incidence and refraction to the refractive indices of two media. It predicts how a ray changes direction at an interface between materials.
Phase velocity: The speed at which a fixed phase point of a wave travels. It is the speed in the denominator of the standard refractive-index definition.
Refractometry: The measurement of a material’s refractive index using how it alters light propagation. It uses index values to identify substances and monitor their composition.
Group velocity: The speed at which a wave packet or modulation envelope propagates. It can differ from phase velocity, so it yields a different index-like measure.
Fresnel equations: Equations describing how light is reflected and transmitted at an interface between media. They show how refractive indices set reflected and transmitted amplitudes.
Vacuum speed of light: The speed of electromagnetic radiation in vacuum, exactly 299,792,458 meters per second. It supplies the reference speed against which a material’s phase velocity is measured.
Lens: An optical element that focuses or diverges light through refraction, reflection, or both. Curved surfaces with different indices redirect rays to form images.
Absorption coefficient: A measure of how rapidly light intensity decreases as it travels through a material. It describes loss of intensity rather than phase propagation, though both relate to complex index.
Optical path length: The distance light would travel in vacuum during the time it takes to cross a medium or path. Refractive index converts physical distance into phase-accumulating optical distance.
Permittivity: A measure of how a material responds to an electric field. For many transparent, nonmagnetic materials, permittivity largely determines the refractive index.

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