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How to calculate real and apparent depth with example

Answer Text:
Real and apparent depth
Consider the following diagram

The depth of the water OM is the real depth, and the distance IM is known as the apparent depth. OI is the distance through which the coin has been displaced and is known as the vertical displacement. The relationship between refractive index and the apparent depth is given by;
Refractive index of a material=real depth/apparent depth
NB
This is true only if the object is viewed normally.
Example
A glass block of thickness 12cm is placed on a mark drawn on a plain paper. The mark is viewed normally through the glass. Calculate the apparent depth of the mark and hence the vertical displacement. (Refractive index of glass =3/2)
Solution
ang= real depth/apparent depth
apparent depth= real depth/ ang=(12X2)/3= 8 cm
vertical displacement= 12- 8=4 cm