Question

while waving your hand very fast in front of your eyes you observe that your fingers appear blurred. What could be the reason for it?

Answer 1

Our retina retains images for 1/16th of a second.  Much like a camera shutter, our eyes don't take perfectly still images.  The light from an object falls on the retina and remains there for some time.  When objects move, light from their new position also falls on the retina.  Usually, these overlap and the object appears perfect without blur.  But if the object is moving very fast, then the light coming from both the locations don't overlap!  This essentially paints two versions of the object.  One in its previous location and other in its new.  These partially overlapped images are perceived by us as blurred.