For light rays travelling from a material with a high index of refraction to a material with a low index of refraction, Snell's law predicts that there is no formula_6 when formula_5 is large. In this case, no transmission occurs; all the light is reflected. This phenomenon is called total internal reflection and allows for fibre optics technology. As light travels down an optical fibre, it undergoes total internal reflection allowing for essentially no light to be lost over the length of the cable.