Refractive Index Calculator
How much a material slows light, both directions.
The definition
n = c / v
Light does 299,792 km/s in vacuum but only 225,000 km/s in water, giving water its index of 1.33. The slowdown at a surface is what bends rays: the straw kinked in the glass, lenses focusing, and rainbows splitting sunlight, since each color slows differently.
Diamond's trick
At n = 2.42, diamond bends light hard and traps it by total internal reflection above a shallow 24° angle, so a well-cut stone bounces light around inside before firing it back at your eye. The sparkle is an equation wearing jewelry.