A shadow is a 'region where light is absent’,  produced by an object that does not allow light to travel through it. Since light travels in a straight path, the light that is impeded by the object results in a black zone with no light, behind the object.

Note: Only when a light source is present, a shadow is created.

Shadows are formed by opaque objects that prevent light from passing through them; transparent objects do not cast a shadow. Translucent objects also cast a shadow but not as dark as opaque objects.

Characteristics of a shadow - Umbra and Penumbra 

if the size of the source of the light is small then only one dark shadow can be observed. This is referred to as the umbra.

If there is a bigger or extended source of light, two shadows are generated, wherein one shadow is darker then the other the darker shadow can be seen at the center while the slightly brighter shadow can be seen along the path of light propagation. These fainter shadows are called Penumbra.

Magnitude of shadow

The magnitude of these shadows is determined by how the screen is positioned in relation to the item. When the screen is moved far away from the subject, the umbra shrinks and the penumbra grows, and vice versa.

Shadow Color.

The absence of light causes the shadow to be black, regardless of the object’s color

Shadow's Length

Light's angle of inctopicIdence determines the length of a shadow. Shadows of people, for instance, decrease in length as the sun rises up.

Eclipse: A shadow forms in space, making the sun or moon invisible for a period of time. Eclipses occur when a bright object is obscured by another object.

Eclipses are classified into three types:

Solar Eclipse:  Solar Eclipse of the sun occurs when the moon passes between the sun and the earth, which results in darkness on the earth (due to the shadow).

Lunar Eclipse: During a lunar eclipse, the sun, moon, and earth are all in line, and the earth is between the sun and the moon, so the sun’s light cannot reach the moon. The earth's shadow falls on the moon, and the moon is not visible.