Refractive index
Beginner
Ratio of light speed in vacuum to that in a medium.
n = c/vn = c/v is greater than 1: air 1.0003, water 1.33, glass 1.5, diamond 2.42. A higher n means slower light and more bending.
Light in glass travels at 2×10⁸ m/s.
n depends on wavelength.
In more detail
The refractive index n = c/v is the ratio of the speed of light in vacuum to its speed in a medium. Water has about 1.33, and glass about 1.5.
Real-world uses
- Gemstones are cut to exploit high refractive indices.
- Contact lens materials are chosen by refractive index.
- Lens designers choose glass with a high refractive index to make thinner and lighter lenses.
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Frequently asked questions
Can refractive index be below one?
Not for a material in the usual sense, since light cannot travel faster than c.
What is the refractive index of diamond?
About 2.42, which gives its brilliance.
Can the refractive index be less than one?
Not for a material in the usual sense, since light is slowest in the densest media, but the phase velocity in some plasmas can exceed c.
Related topics in Optics
Magnification of a mirror · Focal length and radius · Refraction of light · Snell's law · Apparent depth · Critical angle