The Core Role of Refractive Index
The refractive index (n) quantifies how much a material slows down and bends light. It's a dimensionless number that indicates the ratio of the speed of light in a vacuum to its speed within a specific medium. When light passes from one medium to another, the change in its speed, dictated by the refractive indices of the two materials, causes it to change direction or 'bend'.
Impact on Light's Speed and Wavelength
A higher refractive index means light travels slower in that material. For instance, light slows down when it enters water (n ≈ 1.33) from air (n ≈ 1.00). This change in speed also causes a corresponding change in the light's wavelength. While the speed and wavelength of light change as it enters a new medium, its frequency, which determines its color, remains constant.
The Phenomenon of Refraction (Bending Light)
The bending of light, known as refraction, is the primary influence of the refractive index. According to Snell's Law, the angle at which light bends depends on the refractive indices of the two media and the angle at which the light strikes the interface. Light bends towards the normal (an imaginary line perpendicular to the surface) when entering a denser optical medium (higher 'n') and away from the normal when entering a less dense one (lower 'n').
Practical Applications and Dispersion
This influence is fundamental to optical devices. Lenses in eyeglasses, cameras, and telescopes work by precisely bending light using materials with specific refractive indices. Prisms separate white light into its constituent colors (dispersion) because the refractive index of glass varies slightly for different wavelengths of light, bending each color by a different amount. This property is crucial for countless technologies and natural phenomena like rainbows.