In this next diagram I have increased the size of the light source. It is still small relative to our model, so the light is relatively hard, but it is large enough that we can start to see a slight softening of the shadow.
I have placed lines emanating from three points along the face of our light source, one at each edge and one in the center.Looking at the magnified area in the circle we can begin to see how light from the three points interact to create a feathered edge for the shadow. The background area below the red line is illuminated by light rays from all points along our light source. Light from the red point cannot reach the area of the background above the red line, so that area only contains light from the gray and blue points. Light from the gray (center) point cannot reach the background above the gray line, so that area is lit only by the blue point. Now, if you imagine countless lines representing all of the points along the face of the light source, then you can see how the shadow's feathering is the interaction of all of those lines.
In this last diagram I have increased the size of the light source to be larger than the subject. What we see is that there is now much more interaction because light from either extreme of the light source reaches all the way to the center behind our model. This results in a gradual feathering of the shadow that is considered very soft.
An extreme in terms of size, would be the sky on an overcast day, which can leave virtually no shadow.
In the next posting we will take a look at how distance between light source and subject changes the effective size of a light source.
Continue on to part 4"Quality of Light - part 4 (effective size)"