Why is a screen required to see an shadow/image??

Question

why are not we able to see any shadow in the air????

Answer 1

A shadow by itself is simply an absence of light, which can't be seen directly; you can only detect a shadow when there is a nearby light to compare it with.  Air looks clear no matter how much light is shining on it, so there is no contrast between the part of the air that's in light and the part that's in shadow.  When you put up a screen, on the other hand, the part of the screen around the shadow scatters light and appears much brighter than the part covered by shadow, so the shadow can be easily detected by comparison.