Refraction by plane surface and slab

Here the  fig1  shows how rays diverging from a point  O placed in a denser medium, appear to come from lesser distant  point I , if viewed from  rarer medium.

If the point O is at distance d from the boundary AB, the distance of point I will be  d / m .

Fig 2 shows how the rays incident from rarer  medium and  tending to meet at a point O  in the denser medium, gets diverged at the plane boundary   and meet at a distant point I .

If the point O is at distance d from the boundary AB, the distance of point I will be   m d 


Now look at the two  situations shown below where a slab of denser medium is refraction the rays  which are incident from rarer medium. In first case, the rays are tending to converge at some point 

O but are diverged  away  to meet at point I after the refraction from both faces of the slab. This shift from O to I can be obtained as t (1-1/m )   where   t   is the thickness of the slab.

In the other case where the rays are diverging from  a point O, then after refraction by both faces of the  slab, the rays are further diverged and appear to come from  a point    I  which is at distance    t (1-1/m )  again from the point O.

So a  slab of denser medium acts as diverging system !