Question

any one can tell me about denser and rarer midium

Answer 1

denser medium means that the medium which is very much denser than the two medium usually it is glass and the rarer medium is less denser than other medium .
for eg. air with respect to water
water with respect to glass
and others

Answer 2

Optically Rarer medium: when the velocity of light is more then that medium is said to be optically Rarer medium.

Optically Denser medium: When the velocity of light is less when compared to optically Rarer medium then it is said to be optically Denser medium.