The amount of want of illumination in each portion of the penumbra is roughly indicated by the shading.
If we suppose the number of sources to increase indefinitely, so as finally to give the appearance of a luminous surface as the source of light, it is obvious that the degrees of darkness at different portions of the penumbra will also increase indefinitely; i.e.
There will be a gradual increase of brightness in the penumbra from total darkness at the edge next the geometrical shadow to full illumination at the outer edge.
But if we place a body at a distance of a foot or two only from the arc, the shadow cast will have as much of penumbra as if the sun had been the source.
The breadth of the penumbra when the source and screen are nearly equidistant from the opaque body is equal to the diameter of the luminous source.