The ratio of sine of the angle of incidence to the sine of the angle of refraction is a constant for a given colour and for the given pair of media. Which law is this?

This is Snell's Law . Snell's Law states that the ratio of the sine of the angle of incidence (θ1)) to the sine of the angle of refraction (θ₂)) is constant and is equal to the refractive index (n) of the second medium relative to the first medium. Mathematically, it is expressed as: sin θ1 / sin θ₂ = constant This constant is the ratio of the refractive indices of the two media involved. Snell's Law can also be written as: n₁ sin θ₂ = n2 sin θ₂ where n₁ and n₂ are the refractive indices of the first and second media, respectively.