Q.What is principle of reversibility?

The principle of reversibility states that light retraces exactly the same path if its direction of travel is reversed; this holds for both reflection and refraction, and is the geometric reason why, for instance, a ray entering a prism and a ray leaving it obey the same law with the roles of 'incident' and 'emergent' interchanged. In Snell's law, $n_2/n_1$ (written $n_{21}$) is called the refractive index of medium 2 relative to medium 1. This idea gives two useful relations built from Snell's law and reversibility: the inverse rule $n_{12}=1/n_{21}$ (swapping which medium light enters from reverses the ratio), and the chain rule $n_{32}=n_{31}\times n_{13}=n_{31}/n_{12}$ (the refractive index of medium 3 relative to mediu …