The refractive index \(n\) is a dimensionless quantity describing the ratio of the speed of light \(c\) in vacuum to the speed of light \(c_{\text m}\) in a medium:

Formula anchor$$ \begin{align} n ~=~ \frac{c}{ c_{\text m} } \end{align} $$

Here the speed of light in vacuum is a physical constant with the value \( c = 3 \cdot 10^8 \, \frac{\mathrm m}{\mathrm s} \).

So, to determine the refractive index, we have to send the light from the vacuum into a transparent medium (e.g. water, glass) and measure the speed of light in the medium. From the ratio 1 we can calculate the refractive index.

The refractive index \(n\) is larger the slower the light propagates in the medium.

The refractive index \(n\) is smaller the faster the light propagates in the medium. Since the vacuum light speed is the maximum possible speed in the universe, the refractive index cannot become smaller than \(n=1\).

The term " refractive index" comes from the concept of light refraction. When a beam of light from a vacuum enters a medium at an angle, it is deflected (refracted) in the medium.

The refractive index depends on many factors, such as the wavelength of the light, the transparency of the medium, the temperature of the medium, and so on. The following table lists some examples of refractive indices.

Table : Approximate refractive indices of different media with light wavelength 590 nanometers

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