These are formed due to rectilinear propagation of light

Explanation

**Step-by-Step Solution:**

1. **Understanding Rectilinear Propagation of Light:**
- Rectilinear propagation of light refers to the behavior of light traveling in straight lines. This means that when light rays move, they do not bend or curve but instead follow a direct path.

2. **Identifying the Effect of Rectilinear Propagation:**
- When light travels in a straight line, it can be obstructed by objects. This obstruction prevents light from reaching certain areas behind the object.

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