Light and shadows are different and the light does not have a shadow. To make a shadow, you will need any object and a source of light. At night-time, you can also make shadows, but you cannot make a shadow without any light. edit
Umbra and Penumbra edit
The umbra is the darkest region of a shadow. The penumbra is the lightest region of a shadow. The more intense the light source, the harder the shadow and the less variation between umbra and penumbra. The less intense the light source, the softer the shadow and the more variation between umbra and penumbra.
Edges of Shadows edit
Visually, shadows have greatest local value near the edge. This is apparent with more intense light sources. The shadows appear to get darker near the edge of the shadow meeting with the light. This is mostly an illusion. However, when combined with the use of the rendering of umbra and penumbra, shadows in works often have a more interesting purpose.