How Are Shadows Formed?
A shadow forms when light is blocked by an opaque or translucent object. Translucent materials, such as tissue paper, allow partial light through, which scatters and creates a faint shadow. Opaque objects, such as a tree, completely block light and form a dark shadow. Transparent objects, such as glass, do not create a shadow because light passes through in a straight line.
A shadow is cast on the side of the object that is furthest from the light source. The proximity of the object creating the shadow to the light source determines the relative size of the shadow. As an object moves further from a light source, the shadow becomes smaller, and as an object moves closer, the shadow becomes larger. The angle of the light source determines the length of the shadow. At sunrise and sunset, the low angle of the sun creates the longest shadows. The shortest shadows are formed at noon when the sun is highest in the sky.
Most shadows are made up of the umbra and the penumbra. All light is blocked by an opaque object in the umbra. In the penumbra only part of the light is blocked and is an area of partial darkness. For a point source, such as a flashlight shining through a pinhole in a sheet of paper, the shadow that forms is entirely umbra with no penumbra.