A shadow s a dark area where light from a light source is blocked by an opaque object. It occupies all of the three-dimensional volume behind an object with light in front of it. The cross section of a shadow is a two-dimensional silhouette, or a reverse projection of the object blocking the light.
A point source of light casts only a simple shadow, called an “umbra”. For a non-point or “extended” source of light, the shadow is divided into the umbra, penumbra and ant-umbra. The wider the light source, the more blurred the shadow becomes. If two penumbras overlap, the shadows appear to attract and merge. This is known as the Shadow blister effect.
If there is more than one light source, there will be several shadows, with the overlapping parts darker, and various combinations of brightness’s or even colours. The more diffuse the lighting is, the softer and more indistinct the shadow outlines become, until they disappear. The lighting of an overcast sky produces few visible shadows.
For a person or object touching the surface where the shadow is projected (e.g. a person standing on the ground, or a pole in the ground) the shadows converge at the point of contact. A shadow shows, apart from distortion, the same image as the silhouette when looking at the object from the sun-side, hence the mirror image of the silhouette seen from the other side.
The sun casts shadows which change dramatically through the day. Near sunrise and sunset, shadows can be extremely long. Visual artists are usually very aware of coloured light emitted or reflected from several sources, which can generate complex multi-coloured shadows. Chiaroscuro, sfumato, and silhouette are examples of artistic techniques which make deliberate use of shadow effects.
A shadow occupies a three-dimensional volume of space, but this is usually not visible until it projects onto a reflective surface. A light fog, mist, or dust cloud can reveal the 3D presence of volumetric patterns in light and shadow. Theatrical fog and strong beams of light are sometimes used by lighting designers and visual artists who seek to highlight three-dimensional aspects of their work.
The name for the fear of shadows is “Scio-phobia”.