Assertion A mirror has only one focus, while that of lens has two focii.
Reason Both the focii of a mirror coincide at one point.

Options

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

Answer

To solve the question, we need to analyze the assertion and reason provided:

**Assertion:** A mirror has only one focus, while that of a lens has two foci.

**Reason:** Both the foci of a mirror coincide at one point.

### Step-by-Step Solution:

1. **Understanding the Focus of a Mirror:**
- A concave mirror has a single focus point where parallel rays of light converge after reflecting off the mirror's surface. This focus is located along the principal axis of the mirror.

2. **Understanding the Focus of a Lens:**
- A lens, such as a convex lens, has two focal points: one on each side of the lens. When parallel rays of light pass through a convex lens, they converge at the focal point on the opposite side. Similarly, rays coming from the opposite side converge at the focal point on the original side.

3. **Analyzing the Assertion:**
- The assertion states that a mirror has only one focus. This is true for concave mirrors, as they have a single focal point where light rays converge.

4. **Analyzing the Reason:**
- The reason states that both the foci of a mirror coincide at one point. This is also true, as a concave mirror has a single focus, and thus the two foci (if considered) would indeed coincide at that single point.

5. **Conclusion:**
- Both the assertion and the reason are correct. The reason provided is a valid explanation for the assertion, as it clarifies why a mirror has only one focus.

### Final Answer:
Both the assertion and reason are correct, and the reason is the correct explanation for the assertion.

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