For a shadow to form, three essential conditions must be met: a source of light, an opaque object to block the light, and a screen or surface behind the object for the shadow to be cast upon. Shadows are always dark, irrespective of the colour of the object. They only show the outline or shape of the object. The size and shape of a shadow can change depending on the position of the light source relative to the object and the screen. Shadows are formed because opaque objects block light, preventing it from reaching the screen directly behind them.