Assertion (A) : Astigmatism is due to uneven curvature of lens.
Reason (R ) : It is treated with cylindrical lenses.

Options

A. If both assertion and reason are true reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but reason is false
D. If both assertion and reason are false.

Explanation

To solve the question regarding the assertion and reason about astigmatism, we will analyze both statements step by step.

### Step 1: Analyze the Assertion
- **Assertion (A)**: "Astigmatism is due to uneven curvature of lens."
- Astigmatism is indeed a refractive error caused by an irregular shape of the cornea or lens. This uneven curvature prevents light from focusing evenly on the retina, leading to blurred vision. Therefore, this assertion is **true**.

### Step 2: Analyze the Reason
- **Reason (R)**: "It is treated with cylindrical lenses."
- Astigmatism can be corrected using cylindrical lenses, which help to focus light properly onto the retina by compensating for the uneven curvature. This statement is also **true**.

### Step 3: Determine the Relationship
- Now we need to determine if the reason provided is the correct explanation for the assertion.
- While both statements are true, the reason does not explain why astigmatism occurs. The assertion explains that astigmatism is due to the uneven curvature of the lens, while the reason simply states a method of treatment. The actual reason for astigmatism is related to the inability of the optics of the eye to focus light correctly, not just the fact that it can be treated with cylindrical lenses.

### Conclusion
- Since both the assertion and reason are true, but the reason does not correctly explain the assertion, the correct answer is:
- **Both assertion and reason are true, but reason is not the correct explanation of the assertion.**