Refractive Index from Speed Calculator

This calculator finds the refractive index of a material from the speed of light within it, the property that governs how much light bends and slows when it enters that material. Light travels fastest in a vacuum, at very nearly 300 million metres per second, but it slows down in any transparent material, more so in denser ones. The refractive index is simply the ratio of light's speed in a vacuum to its speed in the material, so a higher index means light is slowed more. This single number determines how much a light ray bends as it crosses into the material, through Snell's law, and underlies the behaviour of lenses, prisms, optical fibres and the sparkle of gemstones. This calculator computes it. You enter the speed of light within the medium in metres per second, and the calculator returns the refractive index, the speed in the medium for reference, the vacuum speed of light, and the percentage by which light is slowed. The results update as you type. Use it for optics and physics study, for understanding materials, or for any refraction problem. The refractive index is the speed of light in a vacuum divided by the speed in the medium. Because the vacuum speed is the fastest possible, the index is always one or greater: it is exactly one for a vacuum, very close to one for air, about 1.33 for water, around 1.5 for typical glass, and as high as 2.4 for diamond, whose very high index is what gives it such brilliance and fire. The percentage slowing follows directly: an index of 1.5 means light travels at two-thirds of its vacuum speed in the material, a slowing of about a third. Knowing the refractive index lets you predict refraction angles, critical angles for total internal reflection, and lens behaviour, making it one of the most useful properties of any optical material.

Refractive index = speed of light in vacuum / speed in the medium. Always 1 or more. Water ~1.33, glass ~1.5, diamond ~2.4. Higher index bends and slows light more.