Now, 
The velocity of the band along the column (v) will be the ratio of the column length (L) to the retention time (tr)
Thus, 
Now when 
then the
solute will move through the column in the direction
of the mobile phase and be eluted in the mobile phase at the end of the column.
And when 
then the
solute will move through the column in the opposite
direction to the mobile phase and be eluted by the moving stationary
phase at the beginning of the column.
