A shadow is essentially 2D, but it exists within a 3D space.
Here's a breakdown to explain:
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The 2D Nature of a Shadow: A shadow is fundamentally an area where light is blocked. This area is defined by its length and width, existing on a surface. In this sense, it's like a flat, 2D image projected onto a surface. Think of it as a "sheet" of blocked light.
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The 3D Context: However, this 2D "sheet" is always interacting with 3D objects and existing within a 3D environment. The shape of the shadow is determined by the 3D object blocking the light. Also, the shadow falls on a 3D surface. Therefore, while the shadow itself is essentially 2D, it's inextricably linked to and exists within a 3D space. The reference states: "...a shadow is just like a 2d sheet (of infinitely small thickness) that can be placed on 3dobjects therefore making the shadow exist in 3d."
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Example: Consider a shadow cast by a cube. The shadow's shape (a square, rectangle, or other polygon depending on the light source) is 2D. But the cube is 3D, and the surface the shadow falls on (the ground, a wall, etc.) is also part of a 3D environment.
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Infinitely Small Thickness: As described in the reference material, it might be helpful to think of a shadow as a 2D sheet with an infinitely small thickness. This reinforces the idea that it primarily exists in two dimensions (length and width), even though it's located in 3D space.
In conclusion, while a shadow itself is fundamentally a 2D representation of blocked light, it exists and interacts within a 3D world, being both influenced by and projected onto 3D objects.
