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However, inspection of the diagram shows that the ratio of the length of the smaller hypotenuse to the larger is actually given by the cosine of the angle of refraction divided by sine of the angle of incidence. This is contrary to the 'law of refraction' (often termed Snell's Law) which states that ratio of the sines of the angles of incidence and refraction are equal to the inverse ratio of the refractive indexes. In other words Ibn Sahl's diagram does not anticipate 'the law of refraction' and it cannot be derived from his diagram.