Optically denser and Optically 

rarer medium:

 Case 1:





1) if is the refractive index of a medium w.r.t. an another medium. Again if refractive index is greater than 1, then the second medium is optically denser than the first


 


2) Then 



3) And refracted angle is smaller than the incident angle, that is

i > r  as shown in the diagram




Case 2:


1) If the refractive index of a medium w.r.t. an another medium is smaller than the first, then second medium is lighter than the first medium

 


2) Then



3) In this case the refracted angle is larger than the incident angle

i < r  as shown in the diagram


4) and refracted ray is away from the normal.




Point to be noted: 

1) Due to these deviations in day to day life we experience many surprising facts such as in a tank we see the bottom of the tank rises, when the tank is filled with water.


2) Another very good examples is the total internal reflection. which will be discussed soon. 


3) lighter medium is also called rarer medium