From birth to the age of 2 to 5, the human eye gets trained in the brain to understand the depth of vision. The illusions created by perspective lines are called as "Ponzo- illusion". Relative size is another clue to judge the depth. There is a greater tendency to assume smaller objects than there are in case of larger objects. Superimposition also can create effect of depth. Familiar size is also a strong depth clue. Shadow is an important depth clue.