Snell's law states that sin(i) and sin(r) should be proportional, what two conditions are necessary to establish proportionality?

Snell's law is generally true only when:

1) the beam of light is monochromatic (of one single wavelength, as for example laser beams) 

2) the media through which the light beam travels must be isotropic (uniform in all directions, as for example glass)

In these conditions:
sin (r) / sin (i) = n₁ / n₂ = λ₂ / λ₁