The focal length of a lens is 0.1 m. Then the lens must be

Options

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

Answer

To determine the type of lens based on its focal length, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Focal Length**: The focal length of a lens is the distance from the lens at which parallel rays of light converge (for a convex lens) or appear to diverge from (for a concave lens).

2. **Identify the Given Focal Length**: The problem states that the focal length of the lens is 0.1 m.

3. **Determine the Sign of the Focal Length**: The focal length is given as positive (0.1 m).

4. **Recall Lens Types and Their Focal Lengths**:
- **Convex Lens**: Has a positive focal length. It converges light rays.
- **Concave Lens**: Has a negative focal length. It diverges light rays.
- **Cylindrical Lens**: Typically has a negative focal length as well.

5. **Conclude the Type of Lens**: Since the focal length is positive (0.1 m), the lens must be a **convex lens**.

### Final Answer:
The lens must be a **convex lens**.

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