refractility

Refractility is the property of a material to alter the direction of light as it passes between media of different optical densities. In practical terms, it is described by the refractive index, n, defined as the ratio of the speed of light in vacuum to the speed of light in the material. A higher refractive index means stronger bending of light at interfaces.

The change in light direction is governed by Snell's law: n1 sin(theta1) = n2 sin(theta2). When light moves from a less dense to a more dense medium, theta2 decreases, causing refraction toward the normal; the opposite occurs when moving to a less dense medium. This bending is the basis for lenses and many optical devices.

The refractive index itself varies with wavelength, a phenomenon called dispersion. Different colors bend by different amounts, producing prisms' separation of white light. The refractive index is usually labeled as n(λ), so materials exhibit a spectrum of refractive indices across the visible range and beyond. At normal incidence, light speeds up or slows, but travels straight across an interface without bending; there is still a change in phase velocity.

Applications span optics, photography, astronomy, and telecommunications. Lens design, corrective eyewear, fiber optics, and atmospheric optics rely on known refractive properties. In crystals, anisotropy can produce birefringence, where refractive index depends on polarization direction, a related but distinct phenomenon.

While refractility is commonly described by refractive index, the term appears most often in older or more general texts; in modern usage, refractivity or refractive index is preferred. Quantitative values are given for specific wavelengths and temperature/pressure conditions.