Real depth and apparent depth.
A water tank appears shallower i.e. less deep then what actually is. When we observe an object which is inside the water or any transparent liquid it appear higher then its actual position . This is because when light travels from denser to rarer it bended away from the normal . And we can understand the inverse of this process that when a fish observe an object which is placed above the water level then it appears higher then its actual height from the surface of water this is because when light travels from rarer to denser medium light banded towards the normal.
To watch the video of this topic click on the link given below-
The relation between real depth and apparent with the refractive index is given in point 2 and point 3 below.
After this topic students have to learn Total internal reflection click here to learn- Total internal reflection