The focal length of the eye lens increases when eye muscles

Options

A. Are relaxed and lens becomes thinner
B. Contract and lens becomes thicker
C. Are relaxed and lens becomes thicker
D. Contract and lens becomes thinner

Explanation

To solve the question regarding the focal length of the eye lens and the role of eye muscles, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Role of Eye Muscles**: The eye muscles, specifically the ciliary muscles, control the shape of the eye lens. This is crucial for focusing on objects at different distances.

2. **Identify Muscle Contraction and Relaxation Effects**:
- When the **ciliary muscles contract**, the tension on the lens decreases, causing the lens to become **thicker**. A thicker lens has a **shorter focal length**, which allows for focusing on nearby objects.
- When the **ciliary muscles relax**, the tension on the lens increases, making the lens **thinner**. A thinner lens has a **longer focal length**, which is suitable for focusing on distant objects.

3. **Analyze the Question**: The question states that the focal length of the eye lens **increases**. From our understanding:
- An increase in focal length occurs when the lens becomes **thinner**, which happens when the eye muscles are **relaxed**.

4. **Evaluate the Options**:
- **Option 1**: Eye muscles are relaxed and lens becomes thinner. (This is correct as it leads to an increase in focal length.)
- **Option 2**: Eye muscles contract and lens becomes thicker. (This decreases focal length.)
- **Option 3**: Eye muscles are relaxed and lens becomes thicker. (This decreases focal length.)
- **Option 4**: Eye muscles contract and lens becomes thinner. (This is incorrect as it would also decrease focal length.)

5. **Conclusion**: The correct option is **Option 1**: "Eye muscles are relaxed and lens becomes thinner," which results in an increase in the focal length of the eye lens.

### Final Answer:
The focal length of the eye lens increases when the eye muscles are relaxed and the lens becomes thinner.

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