Shadows is an outline image of an object formed on the opposite side of the light source when light is blocked or partially blocked by an object. The size and shape of a shadow can be changed. Some light sources when blocked by certain object can cast a totally dark shadow, while others form fainter shadow. The sharpness and number of shadows cast by an object depend on the size of the beam of the light source. A small light source, such as a ray of light coming from a small hole, will cast a totally dark and sharp shadow.
A ray of light coming from a small hole
An extended light source or bigger beam of light will form two shadow. The one at the centre being totally dark, while there is a bigger shadow on the outer side which is fainter. The dark shadow with a sharp outline is called umbra while the fainter shadow with a blur outline is called penumbra.
Umbra and Penumbra