This is caused by constructive or destructive interference. Light travels as both waves and packets of energy. The famous light grating experiments show both light and dark spots where 'waves' of light cross. These points can be dark where they cancel each other out or bright where only one wave pattern exists. http://micro.magnet.fsu.edu/primer/lightandcolor/interferenceintro.html Light also "bends" with wavelenghts at different rates...see the rainbow in a prism? As an object obsures light these waves become blocked, some wavelengths more than others, some pass close bye while others are blocked, inference can occur at very smsll or large objects. Thus the outline at a distance is "fuzzy" ... the closer the object the sharper the shadow, the less inference occurs. Outside glane and reflected light also masks the defined lines of a shadow... if this did not exist the shadow directly behind you would be BLACK emptiness with zero light and NO reflections or interference. just blackness...ooooh However we see no of this due to inference & reflectivity, just your shadow along the ground...