According to the principle of reversibility of light, the path of a ray of light is reversible. The figure below shows how light gets refracted from medium 1 to medium 2.
According to Snell's law
Suppose the refraction is taking place from medium 2 to medium 1, then according to the principle of reversibility RO and OI will be the incident ray and the refracted ray respectively.
By Snell's law
From the above relation it is clear that the refractive index of the second medium with respect to the first medium is the reciprocal of the refractive index of the first medium with respect to the second.
