Shadow is formed due to the rectilinear propagation of light.

Rectilinear Propagation of Light:
The rectilinear propagation of light refers to the phenomenon in which light travels in straight lines. When light travels through a homogeneous medium, such as air, it moves in a straight line until it encounters an obstacle or is refracted or reflected. This property of light is the basis for the formation of shadows.

Formation of Shadows:
Shadows are formed when an object obstructs the path of light, preventing it from reaching a surface. The formation of shadows can be explained in the following steps:

1. Source of Light: Shadows are formed when there is a source of light, such as the Sun or a lamp. The light emitted from the source propagates in straight lines.

2. Object: An object is required to obstruct the path of light. When light encounters an object, it interacts with the surface of the object in various ways.

3. Opaque and Transparent Objects: Opaque objects do not allow light to pass through them, while transparent objects allow light to pass through without obstruction. Shadows are formed by opaque objects that block the light completely.

4. Umbra and Penumbra: When light is obstructed by an opaque object, two regions are formed behind the object. The darker, inner region is called the umbra, where the light is completely blocked. The lighter, outer region is called the penumbra, where only a portion of the light is obstructed.

5. Shadow Projection: The formation of a shadow depends on the size and shape of the object, the distance between the object and the surface, and the angle of incidence of light. The shadow is projected onto a surface, such as a wall or the ground, in a shape that is similar to the object.

Conclusion:
Shadows are formed due to the rectilinear propagation of light. When light encounters an opaque object, it is obstructed, resulting in the formation of a shadow. The rectilinear propagation of light ensures that shadows are cast in straight lines, allowing us to observe and study their formation and properties.