What is Refractive index? — Physics

Definition 3.5 High School Physics · Chapter 3 — Refraction of Light

The refractive index of a transparent medium is the ratio

n = \frac{c}{v},

where $c = 3.00 \times 10^{8}\,\mathrm{m}/\mathrm{s}$ is the speed of light in vacuum and $v$ its speed in the medium. Since $v \leq c$ , the index satisfies $n \geq 1$ ; it has no unit. Some useful values:

medium	air	water	plexiglas	glass	diamond
$n$	$1.0003$	$1.33$	$1.49$	$1.50$	$2.42$

Incident (blue), reflected (gray) and refracted (red) rays. All angles are measured from the normal (dashed); entering the denser medium (n_2 > n_1), the ray bends toward the normal. — Incident (blue), reflected (gray) and refracted (red) rays. All angles are measured from the normal (dashed); entering the denser medium ( $n_2 > n_1$ ), the ray bends *toward* the normal.

Examples

Example 3.6 (Speed of light in water)

In water, $v = \dfrac{c}{n} = \dfrac{3.00 \times 10^{8}\,\mathrm{m}/\mathrm{s}}{1.33} = 2.26 \times 10^{8}\,\mathrm{m}/\mathrm{s}$ . In diamond, light crawls at $3.00 \times 10^{8}\,\mathrm{m}/\mathrm{s}/2.42 = 1.24 \times 10^{8}\,\mathrm{m}/\mathrm{s}$ — less than half its vacuum speed. For air, $n = 1.0003$ changes the speed by $0.03\%$ : in this chapter we treat air as vacuum and take $n_{\text{air}} = 1.00$ .

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Definition 2.2 University Physics — Year 1 · Chapter 2 — Geometric Optics: Rays, Reflection, Refraction

A transparent medium lets light through; light travels in it at a speed $v < c$ . Its refractive index is

n = \frac{c}{v} \geq 1 .

Vacuum: $n = 1$ ; air: $1.0003$ ; water: $1.33$ ; ordinary glass: $1.5$ to $1.6$ ; diamond: $2.42$ . A wave of frequency $\nu$ keeps its frequency when it enters a medium, so its wavelength shrinks from $\lambda_0 = c/\nu$ in vacuum to $\lambda = \lambda_0/n$ .

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Examples

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