Assertion : The radius of curvature of a mirror is double of the focal length.
Reason : A concave mirror of focal length f in air is used in a medium of refractive index 2. Then the focal length of mirror in medium becomes 2f.

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 and 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, we need to analyze the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
The assertion states that "The radius of curvature of a mirror is double of the focal length."
- For a spherical mirror (both concave and convex), the relationship between the radius of curvature (R) and the focal length (f) is given by the formula:
\[ R = 2f \]
- This means that the assertion is true.

2. **Understanding the Reason**:
The reason states that "A concave mirror of focal length f in air is used in a medium of refractive index 2. Then the focal length of the mirror in the medium becomes 2f."
- The focal length of a mirror is determined by its shape and does not change with the medium in which it is placed. The focal length is dependent on the radius of curvature and is given by the same formula \( f = \frac{R}{2} \).
- The refractive index affects lenses, but mirrors operate based on reflection, and their focal length remains constant regardless of the surrounding medium.
- Therefore, the reason provided is false.

3. **Conclusion**:
- The assertion is true, while the reason is false. Thus, the correct answer to the question is that the assertion is true, but the reason is false.

### Final Answer:
- Assertion: True
- Reason: False