It is the ratio of the sin of angle of incidence and the sin of angle of refraction ............
    given by      μ=sin i/sin r

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There are two laws of refraction. Since Refractive index is associated with the second law I am mentioning that only. The second law of of refraction states that :
2. For any given pair of media the ratio of sine of angle of incidence and angle of refraction is a constant μ .
         \frac{sin i}{sin r} = μ
The second law is also known as Snell's law. Where μ is the refractive index of the first medium with respect to the second medium. μ is spelled as Mew.
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