Which lens is thicker at the periphery and thinner in the middle

Options

A. convex
B. concave
C. Bi-convex
D. plano-convex

Explanation

To solve the question "Which lens is thicker at the periphery and thinner in the middle?", we will analyze the characteristics of different types of lenses.

### Step-by-Step Solution:

1. **Understand Lens Types**:
- There are two main types of lenses: **convex** and **concave**.
- Convex lenses are thicker in the middle and thinner at the edges, while concave lenses are thinner in the middle and thicker at the edges.

2. **Analyze Convex Lens**:
- A **convex lens** (also known as a converging lens) has a shape that bulges outward.
- This means it is thicker at the center and tapers off to be thinner at the edges.
- Therefore, a convex lens cannot be the answer.

3. **Analyze Concave Lens**:
- A **concave lens** (also known as a diverging lens) has a shape that curves inward.
- This lens is thinner at the center and thicker at the edges.
- Hence, this is not the correct answer either.

4. **Analyze Bi-Convex Lens**:
- A **bi-convex lens** is similar to a convex lens but has two outward curves.
- It is thicker at the center and thinner at the edges, which again does not fit the requirement.

5. **Analyze Pleno-Convex Lens**:
- A **plano-convex lens** has one flat side and one outwardly curved side.
- Like the bi-convex lens, it is thicker at the center and thinner at the edges, making it incorrect for our question.

6. **Conclusion**:
- Since all the options provided (convex, concave, bi-convex, and plano-convex) do not match the description of being thicker at the periphery and thinner in the middle, the correct answer is **a concave lens**.

### Final Answer:
The lens that is thicker at the periphery and thinner in the middle is a **concave lens**.