The refractive index is a dimensionless number that quantifies the bending of light as it passes from one transparent medium into another. It is defined as the ratio of the speed of light in a vacuum to the speed of light in the specific medium. Physically, this index indicates the optical density of the material and is responsible for the phenomenon of refraction, the angular deviation of light rays at the boundary between two substances. Since the index varies depending on the material's chemical composition and the wavelength of light being measured, it is a critical parameter in optics, used to calculate how much the path of light will be altered when passing through interfaces such as glass, water, or air.