What is optically denser and optically rarer medium ?

Explanation

If `n_(21) gt 1` i.e, refractive index of medium-2 is greater than of medium-1, then medium-2 is said to be denser than medium-1.
Snell.s law `(sin i)/(sinr)=n_(21)`
`therefore(sini)/(sin r) gt 1`
`therefore` sin I `gt` sin r
`therefore` i `gt` r
Hence, refracted ray bends toward normal. Here, medium-2 is said to be denser as compared to medium-1.
If `n_(21) lt` 1, i.e. refractive index of medium-2 is less than medium-1, then medium-2 is said to be rarer than that of medium-1.
`(sin" "i)/(sin" "r)=n_(21)` (Snell.s law)
`therefore` sin i `lt` sin r
`therefore` i `lt` r

Hence, refracted rays goes away from normal. Here, medium-2 is said to be rarer as compared to medium-1.