refractive index
/ ree-FRAK-tiv IN-deks /
Stand a straw in a glass of water and it looks snapped in two at the surface. The straw is fine — it's the light that bent on its way from water to air. Light changes speed and direction when it enters a new material, and the refractive index is the single number that says how much.
Precisely, the refractive index measures how much slower light travels inside a material than in empty space, and therefore how sharply it bends when crossing into that material. A vacuum has an index of one; water about 1.33; ordinary glass around 1.5; diamond a hefty 2.4. The higher the index, the more the material drags on the light and the more steeply rays bend at its surface. The slowing happens because the light's electric field jostles the material's electrons, which re-radiate slightly out of step, and the combined wave creeps along more slowly.
Refractive index matters because it is the master parameter of all lens-making, fibre optics, and gem cutting: every spectacle lens, microscope, and camera works by bending light through shaped materials of known index. The honest subtlety is that the index is not a single fixed number for a material — it changes slightly with the color of the light, which is exactly why a prism fans white light into a rainbow and why cheap lenses ring objects with faint colored fringes.
A diamond sparkles so fiercely because its index of 2.4 is among the highest of any clear gem: light entering the stone bends so steeply that it bounces around inside many times before escaping, and along the way the colors split — which is the fire of a well-cut diamond.
Diamond's very high refractive index traps and splits light inside the stone, giving it its fire.
A high refractive index does not mean a material absorbs light or looks dark — diamond and glass are crystal clear yet bend light strongly. Bending (refractive index) and swallowing (absorption) are separate properties.