A watch-maker's glass is a

Options

A. convex lens
B. concave lens
C. plano-convex lens
D. Plano-concave lens

Answer

To answer the question "A watch-maker's glass is a," we will analyze the options provided and determine the correct type of lens.

### Step-by-Step Solution:

1. **Understanding the Purpose of a Watchmaker's Glass**:
- A watchmaker's glass is used to magnify small parts of watches. This allows watchmakers to see intricate details that are not visible to the naked eye.

2. **Identifying the Type of Lens**:
- There are different types of lenses: convex, concave, plano-convex, and plano-concave.
- A **convex lens** is a converging lens that can magnify objects. It is thicker in the center than at the edges.

3. **Evaluating the Options**:
- **Convex Lens**: This lens type is suitable for magnification. It can make small objects appear larger.
- **Concave Lens**: This lens diverges light rays and is used for correcting nearsightedness, not for magnification.
- **Plano-Convex Lens**: This is a type of convex lens, but it is specifically designed with one flat side and one curved side.
- **Plano-Concave Lens**: This lens diverges light and is not used for magnification.

4. **Conclusion**:
- Since the primary function of a watchmaker's glass is to magnify small details, the correct answer is **Convex Lens**.

### Final Answer:
A watch-maker's glass is a **Convex Lens**.