10)] that if the relative refractive index of the
refracting medium is less than one then, since the maximum value of sin
r is unity, there is an upper limit to the value of sin i for which the law
can be satisfied, that is, i = ic such that
sin ic  = n 21
(9 12)
For values of i larger than ic, Snell’s law of refraction cannot be
satisfied, and hence no refraction is possible The refractive index of denser medium 1 with respect to rarer medium
2 will be n12 = 1/sinic Some typical critical angles are listed in Table 9