What is the principle of reversibility of light?

The principle that if a beam of light is reflected back on itself, it will traverse the same path or paths as it did before reversal.

The principle of reversibility states that light will follow exactly the same path if its direction of travel is reversed.

Hence:

Using Snell's Law,

sin i

sin r

= 1n2

sin r

sin i

= 2n1

1n2=1

2n1

It follows that

1n2=1

2n1