Which mirror is used as a back mirror in motor vehicles ?

Options

A. Plain
B. Circular
C. Concave
D. Convex

Explanation

**Step-by-Step Solution:**

1. **Understanding the Purpose of the Back Mirror:**
- The back mirror in motor vehicles is designed to provide the driver with a view of the traffic behind them. This is crucial for safe driving, especially when changing lanes or reversing.

2. **Identifying the Types of Mirrors:**
- There are different types of mirrors: plain (flat), circular, concave, and convex. Each type has unique properties regarding how they reflect light and form images.

3. **Analyzing the Requirements for the Back Mirror:**
- The back mirror should have a wide field of view to allow the driver to see more area behind the vehicle.
- It should produce a smaller image of the objects behind, which helps in viewing a larger area.

4. **Evaluating the Types of Mirrors:**
- **Plain Mirror:** Provides a true-size image but has a limited field of view.
- **Concave Mirror:** Can magnify images but has a narrow field of view and is not suitable for this purpose.
- **Convex Mirror:** Curved outward, it has a wide field of view and produces smaller images of objects, allowing the driver to see more of the area behind the vehicle.

5. **Conclusion:**
- Based on the analysis, the mirror that meets the requirements for a back mirror in motor vehicles is the **convex mirror**. It provides a wide field of view and allows the driver to see far-off traffic effectively.

**Final Answer:**
The mirror used as a back mirror in motor vehicles is a **convex mirror**.

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