The term “shadow” refers to a wide range of light intensity conditions – it is not darkness. A shadow is where there is a difference between a specific light intensity and a specific lower light intensity next to it. The difference in light intensity between the two light levels creates a contrast that our eye can see.
Shadow is created by an object intercepting light from a light source. Any light that can pass the object will be brighter than the light behind the object where the beam has been blocked. The edges of shadows are the defined differences in contrast between two different light intensities.
The greater the difference in light intensity between a light area and a shadow area the higher the contrast between the two. In a very hard light source where there is little reflected light nearby an object will tend to cast a dense shadow with sharp lines defining its edges.
In a soft light the edges of the shadows will be less well defined because the beam of light comes from a diffused light source. The intensity of a shadow will also be reduced where reflected light bounces back into the shadow area and thus reduces the contrast between the light in the beam and the light in the shadow.