Shadows do not need light to form although most shadows are formed by light going past the edges of the material which make the outline for the shadow.
Shadows work by way of the light rays going past the edges of the material or opaque object.
The rays of light do not pass through the object but instead go past the edges of the material.
Rays of light are what creates shadows.
When an opaque object or material is placed in the path of rays of light a shadow is formed.
The opaque material however does not let the light pass through it and instead the lights rays that go past the edges of the object or material make an outline for the shadow.
The three things that are needed to form or create a shadow are.
A source of light.
An opaque object whose shadow has to be formed.
A screen on which shadow is to be formed.
When light reaches an opaque (solid) object, the light is prevented from moving through the object, leaving an area of darkness on the other side (a shadow).
If an object is transparent, the light can move through it.
If the object is shiny, the light will be reflected off of it into a different direction.
Transparent objects cannot form a shadow.
Shadows are only formed in opaque or translucent objects.
If you take a glass slab, no shadow is formed because it allows light to pass through it.
A shadow is a dark area where light from a light source is blocked by an 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.