The focal length of a lens is -0.4m. The lens is

Options

A. convex
B. concave
C. cylindrical
D. none of these

Explanation

To determine the type of lens based on the given focal length of -0.4 m, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Focal Length**:
The focal length (f) is given as -0.4 m.

2. **Understand Focal Length Sign Convention**:
In optics, the sign of the focal length helps to identify the type of lens:
- A positive focal length indicates a **convex lens** (converging lens).
- A negative focal length indicates a **concave lens** (diverging lens).

3. **Apply the Sign Convention**:
Since the given focal length is negative (-0.4 m), we can conclude that the lens is a concave lens.

4. **Conclusion**:
Therefore, the lens with a focal length of -0.4 m is a **concave lens**.

### Final Answer:
The lens is a **concave lens**.

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